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Organisms and food webs in rock pools: Responses to environmental stress and trophic manipulation Marie Arn6r Department of Zoology, Stockholm University S-106 91 Stockholm, Sweden Stockholm 1997 Doctoral dissertation 1997 Marie Am& Department of Zoology Stockholm University S-106 91 Stockholm [email protected] 0 1997 Marie Am& ISBN 9 l-87272-53-9 Printed by Jannes Snabbtryck AB, Stockholm Cover by Bibbi Mayrhofer 2 ERRATA Organisms and food webs in rock pools: Responses to environmental stress and trophic manipulation by Marie Arner page line written Summary 6 9 (1992) Physiological and life history responses of Duphnia magna to increasing salinity (submitted manuscript) 6 13 10 10 11 13 23 right 20 Koehn & Bayne 1988 right 24 Koehn & Bayne 1988 right 32 ..physiological index. (.. left 21 (1996) has argued... left 22 Koehn R.K. &‘Bayne, B.L (1988) Paper III 4 left 18 . ..The water was sieved.. right 41 and nauplia to Cyclops 6 Paper IV 12 left 21 . . . Daphnia generally had lower biomass... Paper V right 8 of salinity tolerance of the 3 DaphnM right 24 The ratios of nauplia to Cyclops 5 should be (1993) Effects of salinity on metabolism and life history characteristics of Daphnia magna (accepted for publication in Freshwater Biology) Koehn & Bayne 1989 Koehn & Bayne 1989 ..physiological index (... (1996) have argued... Koehn R.K. & Bayne B.L (1989) The water (50 L tub-‘) was sieved and nauplia to total Cyclops . . . Duphnia generally had higher biomass.. . of salinity tolerance of Daphnia The ratios of nauplia to total Cyclops To my family Abstract Differential susceptibility of organisms and populations to environmental stress influences the outcome of biological interactions and the structure of communities and ecosystems. In this thesis, the effects of environmental stress on organism, population and community levels were studied. ‘Die physiological responses to changes in salinity and exposure to pollutants were studied by comparing rock pool Gammurus duebeni and littoral G. oceanicus with different tolerance to abiotic stress. Physiological and life history responses of rock pool Duphnia magna to different salinities were examined. Ex-, perimental systems, originating from natural rock pools, were established to explore direct and indirect impacts of cadmium and predator addition in freshwater plankton communities. Three different food web configurations were used: 1. phytoplankton and small-bodied zooplankton (Cyclops sp. and Chydorus sphaericus), 2. phytoplankton, small-bodied zooplankton and D. magna, and 3. phytoplankton, small-bodied zooplankton, D. magna and the invertebrate predator Notonectu sp. To evaluate the experimental systems, natural and experimental rock pools were compared. Salinity stress negatively affected the physiological status of Gammurus and D. magna. G. duebeni, with higher tolerance to fluctuation in abiotic variables, was less affected by natural stress and pollutants than G. oceanicus. The physiological and life history responses led to comparable conclusions in D. magna: i.e., salinity stress negatively affected the physiological status of D. mugnu and hampered reproduction and growth. In the experimental food webs, cadmium inhibited phytoplankton productivity and decreased the biomass of cladocerans. Cadmium did not change the trophic interactions between Duphniu and phytoplankton or between Duphniu and Notonectu. The regulation of lower trophic levels by Duphniu and Notonecta was important in the experimental food webs. Notonectu produced a indirect positive (cascade) effect on phytoplankton and small-bodied zooplankton. It was possible to maintain experimental phytoplankton-herbivore communities for several months. The experimental systems resembled natural rock pools with permanent D. mugnu presence. Phytoplankton biomass was regulated by D. magna when the species was permanently present in both natural and experimental rock pools. Experimental rock pools may approximate other fishless habitats and the spatial and temporal scales are most appropriate for studies of plankton interactions. 4 Organisms and food webs in rock pools: Responses to environmental stress and trophic manipulation Akademisk avhandling som for avlaggande av filosofie doktorsexamen vid Stockholms Universitet offentligen forsvaras torsdagen den 29 maj 1997, kl. 10.00 i fiirelfsningssalen, Frescati Backe, Svante Arrhenius vag 21 A, Frescati MarieavArnCr Zoologiska Institutionen Stockholms Universitet S-106 91 Stockholm Stockholm 1997 ISBN 9 l-87272-53-9 Abstract Differential susceptibility of organisms and populations to environmental stress influences the outcome of biological interactions and the structure of communities and ecosystems. In this thesis, the effects of environmental stress on organism, population and community levels were studied. The physiological responses to changes in salinity and exposure to pollutants were studied by comparing rock pool Gummurus duebeni and littoral G. oceanicus with different tolerance to abiotic stress. Physiological and life history responses of rock pool Duphniu magna to different salinities were examined, Experimental systems, originating from natural rock pools, were established to explore direct and indirect impacts of cadmium and predator addition in freshwater plankton communities. Three different food web configurations were used: 1. phytoplankton and small-bodied zooplankton (Cyclops sp. and Chydonrs sphuericus), 2. phytoplankton, small-bodied zooplankton and D. magna and 3. phytoplankton, small-bodied zooplankton, D. mugnu and the invertebrate predator Nofonectu sp. To evaluate the experimental systems, natural and experimental rock pools were compared. Salinity stress negatively affected the physiological status of Gummunrs and D. mugnu. G. duebeni, with higher tolerance to fluctuation in abiotic variables, was less affected by natural stress and pollutants than G. oceunicus. The physiological and life history responses led to comparable conclusions in D. magna: i.e., salinity stress negatively aflected the physiological status of D. magna and hampered reproduction and growth. In the experimental food webs, cadmium inhibited phytoplankton productivity and decreased the biomass of cladocerans. Cadmium did not change the trophic interactions between Duphniu and phytoplankton or between Duphniu and Notonectu. The regulation of lower trophic levels .by Duphniu and Notonectu was important in the experimental food webs. Notonectu produced a indirect positive (cascade) effect on phytoplankton and smallbodied zooplankton. It was possible to maintain experimental phytoplankton-herbivore communities for several months. The experimental systems resembled natural rock pools with permanent D. magna presence. Phytoplankton biomass was regulated by D. mugnu when the species was permanently present in both natural and experimental rock pools. Experimental rock pools may approximate other fishless habitats and the spatial and temporal scales are most appropriate for studies of plankton interactions. Table of contents List of papers Introduction The rock pool ecosystem Environmental stress - Background - Rock pool organisms and effects of salinity changes and toxicants 10 12 Model systems and experimental rock pools - Background - The experimental rock pool systems: experience and evaluation 13 13 Consumer and resource regulation - Experience from lake and experimental studies - Predation and competition in rock pool systems - Consumer regulation of lower trophic levels by Notonecta and Daphnia in experimental rock pools - Effects of cadmium addition on trophic interactions in rock pool food webs 15 16 17 18 Conclusions 18 References 19 Acknowledgements 27 5 List of papers The following papers are included in this thesis and will be referred to in the text by their Roman numerals. The published and accepted papers are reprinted with kind permission of the publishers. . I Tedengren M., Am& M. & Kautsky N. (1988) Ecophysiology and stress response of marine and brackish water Gammarus species (Crustacea, Amphipoda) to changes in salinity and exposure to cadmium and diesel-oil. Mar. Ecol. Prog. Ser. 47:107-l 16 II ArnCr M. & Koivisto S. (1992) Physiological and life history responses of Daphnia magna to increasing salinity. Hydrobiologia 259: 69-77 III ArnCr M., Koivisto S., Norberg J. & Kautsky N. Trophic interactions in rock pool food webs: regulation of zooplankton and phytoplankton by JVutonecta and Daphnia. (submitted manuscript). IV Koivisto S., ArnQ M. & Kautsky N. (1997) Does cadmium pollution change trophic interactions in rockpool food webs? Accepted for publication in Environ. Toxicol. Chem. V Am& M. & Koivisto S. Evaluation of the ecological relevance of a model system: Seasonal development and patterns in natural and experimental rock pools (manuscript). 6 & Nuutinen 1985; Ranta et al. 1 9 8 7 ; Ranta & Espo 1989; Pajunen & Salmi 1991; III). The objectives of this thesis can be seen in both a basic ecological and an ecotoxicological context. Organisms and populations have different susceptibility to environmental stress (natural and antropogenic), which influence the outcome of biological interactions and the structure of communities and ecosystems. The effects of salinity changes and pollutants have been studied on organism, population and community levels. Experimental systems, originating from rock pools, were established to explore the effects of trophic manipulations and the addition of a toxicant in a freshwater planktonic community. In paper I, the physiological responses to changes in salinity and exposure to cadmium and diesel-oil were studied by comparing rock pool and littoral amphipods with different tolerance to abiotic stress (Gammanzs duebeni Lilljeborg and G. oceanicus Segerstrlle, respectively). Physiological and life history responses of rock pool Duphnia magna Straus to different salinities were examined in paper II. Paper III comprises studies of the regulation of lower trophic levels by D. mugnu and an invertebrate predator, the backswimmer Notonecta sp. in the experimental rock pools. The direct and indirect impacts of cadmium addition on trophic interactions were also studied in the experimental food webs (IV). The last paper (V) presents and compares the seasonal development in natural and experimental rock pools and evaluates the relevance of the experimental rock pool systems for ecological studies. Introduction Rock pools, water filled bed-rock depressions, are patches of habitat different from the surrounding shore arid are found on shores around the world. They are characterised by large spatial and temporal variations and are considered as physically harsh habitats (Ganning 197 1; Ranta 1982; Astles 1993; Metaxas & Scheibling 1993; Loder et al. 1996; Underwood & Skilleter 1996). These isolated habitats are commonly found along the Swedish and Finnish coastal areas of the Baltic Sea. Earlier studies of rock pools in the Baltic Sea area have emphasised species composition and abundance in relation to the physicochemical characteristics (in particular salinity) of the rock pools (Levander 1900; Jarnefelt 1940; Lindberg 1944; Droop 1953; Ganning 1967; Bjorklund 1972; Ranta 1979). Descriptive studies of different species and of physiological tolerance to normally fluctuating variables and toxicants have been conducted by Forsman (195 l), Lagerspetz (1955, 1958) (I, II) and the nutrient and metabolic dynamics of rock pool ecosystems were studied by Ganning & Wulff (1969, 1970) and Ganning (1971). The systems have also been used for modelling approaches and studies of population dynamics (Wulff 1980; P a j u n e n 1 9 9 0 ; Norberg & DeAngelis 1997). Commonly studied are different aspects of species coexistence and interspecific competition (Vepsalainen 1 9 7 8 ; Ranta 1979, 1982; Hanski & Ranta 1983; Bengtsson 1988) and the effects of vertebrate and invertebrate predation have been studied in situ as well as in laboratory experiments (Ranta 7 The rock pool ecosystem pools means few microhabitats and also large and rapid fluctuations in abiotic variables. The potential number of species will thus be low. Salinity has a significant influence on the distribution of planktonic algae, flagellates, daphnids and rotifers in rock pools (J&rnefelt 1940; Droop 1953; Lagerspetz 1955; Bjorklund 1972; Ranta 1982; V). However, many of the inhabitants in rock pools tolerate greater fluctuations in salinity and other abiotic variables than those normally found (see e.g Forsman 195 1; Ganning 1967; 197 1). The habitat isolation makes dispersal more difficult and restricts colonisation. Local extinctions of species in individual rock pools are common, whereas the species are present on a regional scale (within and among islands) (Hanski & Ranta 1983; Bengtsson 1988). The formation of dormant stages is a common strategy in physically harsh and unpredictable environments (Hairston Jr. 1987). This is common among zooplankton and many other groups of rock pool species (Ranta 1982). Water insects (imagines) and gammarids may escape when conditions deteriorate while other species survive drought or freezing in the sediments. Other adaptations to unpredictable habitats are short life cycles, rapid development to maturity and parthenogenetic reproduction. Living in ephemeral or stressful habitats like rock pools may also offer advantages as predators are relatively rare (Ganning 1971; Sih 1987; Ranta & Espo 1989). This thesis mainly deals with the ecosystem of permanent freshwater rock pools (sensu Ganning 1971). The studies of natural rock pools have been carried out in the Askij area in the northern Baltic proper (Fig. 1). The distinctions between different pools are not absolute and permanent pools of intermediate Scandinavian rock pools have been classified in different ways, for example according to salinity, distance from the shoreline, vegetation, area or durational stability (permanent vs. ephemeral) (Levander 1900; Forsman 195 1; Ganning 1971). The volume of rock pools ranges from a few litres to a few cubic meters with weekly to monthly variations dependent on season, volume, surface to volume ratio and the amount of vegetation (Ganning & Wulff 1970; Ranta 1982; Bengtsson 1988). Rain, evaporation, sea spray and to lesser extent freezing regulate the salinity of the rock pools. Annual salinity in medium sized (= lm3 ) freshwater and brackish water pools normally fluctuates between 0 4%0 S and 3 - 8 %O S, respectively (Ganning 1971; V). High concentrations and large daily and seasonal fluctuations of nutrients are common, reflecting the biotic activities within pools as well as the importance of nutrient additions by rain water run off (Ganning & Wulff 1969; Wulff 1980; V). Rock pools ate rarely anoxic, but large daily variations in dissolved oxygen concentrations occur, as a consequence of primary production and respiration in the pools. Normal daily oxygen saturation levels in freshwater and brackish water rock pools fhrctuate between 50 - 150% and 20 - 200 %, respectively (Ganning & Wulff 1970; personal observations). The primary productivity is usually high and autotrophic conditions generally prevail during the summer season (Ganning & Wulff 1970). The food webs of rock pools am simple and the number of species generally increases with increasing pool size (Ranta 1982). The small size of rock 8 Fig. 1 The Baltic Sea and the study area (Asko). freshwater - brackish water character have also been studied (V). Typical freshwater pools are situated high up on the shore and have limnetic character. Most of the organisms are which planktonic facilitates sampling (Wulff 1980). Primary producers are pelagic and benthic microalgae but also filamentous species occur (Droop 1953; Ganning 1971; Hallfors 1984; Ranta et al. 1987, V). Chironomid larvae and ostracods often live associated with benthic parts of the pools. Common zooplankton are daphnids (D. magna, D. longispina, D. pulex), Chydorus sphaericus, cyclopoid copepods and rotifers. D. magna generally dominates the zooplankton community in freshwater rock pools (Ranta & Espo 1989; V). This species is widely spread geographically, but the large size (adult 9 omnivorous (Forsman 195 1; personal observations). The species hibernates in the bottom substratum and may migrate over the rock between pools (Forsman 195 1; Ganning 1971). Vertebrate predators are absent in most rock pools. To be omnivorous or to have several different types of prey, is advantageous in the limited and unpredictable rock pool habitat. females 5-6 mm) restricts the disitribution to smaller habitats (eutrophic ponds, bog lakes, rock pools), devoid of planktivorous fish (Hebert 1978). D. magna is an efficient generalistic filterfeeder and may ingest food particles ranging from bacteria to microplankton (diameter of 0.2 to 150 l.trn) (Scavia and Fahnenstiel 1988; Stockner and Porter 1988; Lair 1991; Kerfoot and Kirk 1991). D. magna tolerates low oxygen conditions, high pH and wide ranges of salinity and temperature (Kobayashi dz Gondi 1985; MacIsaac ef al. 1985; II). When conditions deteriorate, reproduction changes from parthenogenetic to sexual and resistant ephippial eggs are produced. D. magna is thus well adapted for the unpredictable and variable rock pool habitat. Migration between pools occurs most likely by passive transport of ephippia by water, wind or birds (Ranta 1979). The small size, reproduction, high parthenogenetic fecundity, short life span and easy handling in the laboratory have promoted the use of D. magna in aquatic toxicity testing, although its ecological relevance has been questioned (Koivisto 1995). Other consumers are represented by water insects (i.e. corixid species, diving beetles) and the amphipod Gammarus duebeni (Ganning 1971; Pajunen 1977; Ranta 1982; Bengtsson 1988). The amphipod G. duebeni is the only species occupying all types of rock pools around the Swedish coast, but is practically never found in the littoral zone (Forsman 195 1; Ganning 197 1). Compared to other Baltic Gammarus species, its physiological capacities are well developed to cope with fluctuations in salinity, dissolved gases and temperature (Bulnheim 1984). It is most abundant in brackish-water and freshwater pools and is Environmental stress Background Environmental stress is here defined as an environmental change that results in a reduction of net energy balance and production (i.e. growth and reproduction) (Koehn & Bayne 1988). If the net energy balance of the organism is reduced as a response to environmental stress (natural or antropogenic), it can be assumed that fitness is reduced (Koehn & Bayne 1988). The net energy balance decreases if the inputs are decreased by lower feeding rates or assimilation efficiency and/or if the energy losses are increased by higher respiration and excretion rates. Differential susceptibility of organisms and populations to environmental stress influence the outcome of biotic interactions and the organisation of communities and ecosystems (Dunson & Travis 1991). However, it is difficult to predict the outcome of environmental stress on interspecific interactions, community structure and ecosystem function by extrapolating the results from single-species and life history experiments (Kimball & Levin 1985; Leblanc 1985; Levin et al 1 9 8 9 ; Clements & Kiffney 1994). To resolve this problem, different types of multispecies model systems have been 10 developed (see e.g. Taub & Cow 1980; Borgmann et al. 1988; Heimbach et al 1992, IV). The responses to salinity changes and exposure to cadmium and diesel-oil is focused in this thesis. Organisms living in rock pools (e.g Gammarus duebeni and Daphniu magna) are frequently confronted with rapid and large changes of several abiotic variables and their physiological capacities are well developed to cope with irregular fluctuations in salinity, temperature and dissolved gases (Bulnheim 1972, 1979; 1984; Kobayashi & Gondi 1 9 8 5 ; MacIsaac et al. 1985; I, II). Deviations from normal salinities influence the metabolic rates in invertebrates. Adaptations to subnormal salinities and life in freshwater involve development of more efficient mechanisms for water elimination and salt uptake, as well as reduced surface permeability and increased capacities of salt retention (Kinne 1964). Euryhaline species, e.g. the genus Gammurus, have usually higher oxygen consumption and higher ammonium excretion rates in subnormal salinities (Kinne 1964; Spaargaren 1984; I). Reduced numbers of hatched eggs have been found for G. u’uebeni in freshwater as compared to brackish water (Kinne 1964). Reduced reproductive performance and m final size have also been reported in freshwater animals penetrating into a saline environment (e.g. D. magna) (Kinne 1964; Cowgill & Milazzo 1990, Sublethal concentration of 199 1). cadmium have been shown to reduce oxygen consumption in marine crustaceans (Thurberg et al. 1973). Increased protein turn-over and associated increases in respiration have been found for D. mugnu when exposed to chronic cadmium stress (Barber et al. 1990). It has been suggested that species which tolerate a high degree of natural abiotic stress are more tolerant to pollution stress as well (Fisher 1977; Leblanc 1985; paper I). The 0:N ratio, that describes the relative proportions of oxygen consumed to nitrogen excreted, has been used to describe the physiological status of invertebrates (Bayne ef al. 1985; Mayzaud dz Conover 1988; Tedengren 1990; I, II). This index reflects the balance between fat, carbohydrate and protein substrates in the metabolism (Mayzaud & Conover 1988). There is empirical evidence that a lowering in the 0:N ratio is an indication of environmental stress (W&lows & Phelps 198 1; Carr & Linden 1984; Axiak & George 1987; Tedengren 1990). The nitrogen part of the index is usually estimated by the excreted ammonium, which is the dominant released nitrogen component in crustaceans. 80 - 90 % of the total released nitrogen in both Gammarus and Daphnia magna is composed of ammonium (Sutcliff 1984; Mayzaud & Conover 1988; Urabe 1993). However, respiration and excretion rates of soluble nitrogen may be affected by other factors, e.g. food abundance (starvation), food quality and body size, which add uncertainty to the use of the physiological index. (Conover & Comer 1968; Lampert dz Bohrer 1984; Ejsmont Karbin 1984; Mayzaud & Conover 1988; Urabe 1993). The correspondence of the physiological index and life history characteristics of D. magna exposed to different salinities was studied in paper II. Environmental stress also affects the relative abundance of organisms in aquatic ecosystems both directly and indirectly. Increased mortality or decreased reproduction are examples of direct negative effects and the indirect effects arise when species have different tolerance 11 to toxicants (Koivisto 1996). For example, if the predator is relatively more susceptible to environmental stress, the prey species may experience a positive indirect effect of reduced predation pressure. On the other hand, if the prey species are less tolerant, the predator is negatively affected as the food supply is reduced. Cladocerans are among the most susceptible species to various contaminants and a common indirect effect of pollution is a decline in cladoceran abundance followed by increases in phytoplankton and rotifer abundance (Hurlbert et al. 1972; Hodson et al. 1979; Hamilton et al. 1988; Borgmann et al. 1989; Hanazato & Yasuno 1990; Webber et al. 1992; Havens 1994). cadmium in the rock pool species. The Baltic populations of G. duebeni and G. oceanicus were generally more sensitive to salinity changes and treatments with diesel oil and cadmium than North Sea conspecifics. The higher sensitivity to pollutants of the Baltic populations may be due to a number of factors such as changes in the characteristics of toxic substances (metals) with salinity, the higher relative ionic concentration of a given amount of poisonous substance in the low saline Baltic Sea water as compared to the North Sea water, and direct interactions of toxicants with membrane permeability and osmoregulatory mechanisms, which are already under strain at low salinities. A majority of cladoceran species am exclusively freshwater animals, although a few genera have colonised saline environments. Daphnia magna is commonly found in brackish water rock pools and salinity is known to affect the distribution. In paper II we investigated the effects of salinity on metabolism and life history characteristics of rock pool D. magna. The lowest 0:N ratio were found at 8%0 S (as compared to freshwater and 4%0 S), indicating that the most stressful conditions prevailed at the highest salinity. Based on the 0:N ratio, the physiologically most favourable conditions were at 4%0 S. The life history characteristics showed that the most suitable salinity for growth and reproduction of D. magna was 4%0 S, whereas the lowest population growth rate was obtained at the highest salinity. The results showed that high salinity affected the physiological status of D. magna negatively, hampering reproduction and growth. The physiological index and the life history variables led to comparable conclusions, although the life Rock pool organisms and eflects of salinity changes and toxicants The effects of environmental stress on the metabolism of one rock pool species (Gammurus duebeni ) and one littoral species (G. oceanicus) were studied in paper I. The two Gammarus species am common along the whole Swedish coast, and organisms from both marine (North Sea) and brackish water (Baltic Sea) areas were studied. Both species are adapted to environmental variable conditions, characteristic of both rock pools and the littoral zone. However, G. duebeni is more tolerant to fluctuations in e.g. salinity, temperature and oxygen levels than other Baltic Gummurus species, and was regatded as a relatively more broadniched species (Bulnheim 1984). The rock pool species was expected to he more tolerant to both natural and maninduced stress. The results from the laboratory experiments supported this hypothesis, and respiration and the 0:N ratio were less affected by salinity, changes and/or additions of diesel-oil and 12 generally covered as many or more generations of the selected organisms. There are obvious advantages with model systems. The input and output of energy, material and organisms can be controlled and the small scale and simple structure facilitates replication, repetition and sampling. Some of these positive features are at the same time valid ground for criticising of model systems. It has e.g. been claimed that model systems are generally too simple in structure and assembled of species lacking shared evolutionary history. Furthermore, there are difficulties in extrapolating the results from studies of limited model ecosystems to real world effects, due to e.g. the lack of seasonality or density-independent disturbances. Despite this criticism, them is a general agreement that model systems may offer a bridge between the simplicity of mathematical models or small scale experiments and the full complexity of real systems (Lawton 1996, Drake et al. 1996, Verhoef 1996). Carpenter (1996) suggested that the main role of microcosms are supportive to field studies and that they can be used to eliminate hypothesised mechanisms, compare alternative mechanisms or estimate rates. The model ecosystems ate suitable for ecotoxicological studies as the interactions between species, indirect effects of toxicants and the impacts of abiotic factors on toxicity of chemicals can be studied without damaging the environment. history characteristics appeared to be more sensitive indicators of the most favourable treatment. Model systems and experimental rock pools ’ Background The opinions of what is the most relevant temporal and spatial scales and experimental methods for studying community and ecosystem ecology diverge (Lawton 1996, Drake et al. 1996, Carpenter 1996). The realism and relevance of natural systems are indisputable, but the sometimes overwhelming complexity complicates the understanding and interpretation of patterns and processes. Drake et al. (1996) has argued “that this inherent complexity of nature is what makes the use of laboratory microcosms both necessary”. valid and desirable, Experimental systems (micro- and mesocosms) are models of natural ecosystems, including a selected part of the biological processes and interactions that occur in the field. Model ecosystems should be isolated, self-maintaining, include more than one trophic level and maintain the same functions as the part of the ecosystem which it is supposed to represent (Lalli 1990). The spatial scale of model systems range from indoor laboratory bottles to large outdoor enclosures of nature. Most work has focused on smaller organisms with short generation time such as bacteria, soil organisms, protists or plankton (Drake et al. 1996, Verhoef 1996). In a survey of articles treating species interactions, Ives et al. (1996) found that microcosm studies generally were shorter than field studies measured in real time, but The experimental rock pool systems: experience and evaluation Trophic interactions and the regulation of phytoplankton and zooplankton were studied in experimental freshwater systems with three food web configurations: 1. phytoplankton and small- 13 bodied zooplankton, 2. phytoplankton, small-bodied zooplankton and Duphniu magna and 3. phytoplankton, smallbodied zooplankton, D. mugnu and the backswimmer Notonecta sp. (III, IV). The small-bodied zooplankton mainly consisted of Cyclops sp. and Chydorus sphaericus. Water and plankton organisms originated from freshwater rock pools. The outdoor experimental systems were established in 55 L plastic tubs with sand and small stones as bottom substrate. Water, phytoplankton, zooplankton and Notonecta were added successively to allow stabilisation of the systems. Cadmium and Notonecta were &led six weeks after water addition and the experiment was terminated eight weeks later because of high Notonecta mortality. The experimental systems had probably too low productivity or were too small to support a third trophic level during long term experiments. The experiment used two cadmium levels: controls (no cadmium) and the nominal concentration of 20 ppb; in all six treatments with five replicates each. The experiment was static, with no water renewal, but cadmium was replenished when concentrations fell below the desired concentration. About once a week, the experimental rock pools were sampled for the estimation of phosphate and ammonium concentrations, phytoplankton biomass, primary productivity, zooplankton species composition and biomass. Ammonium was analysed as it is generally the main inorganic nitrogen compound in rock pools (Wulff 1980). The results are presented in two parts; firstly the effects of Notonecta and D. magna on lower trophic levels and nutrient concentrations (III) and secondly the combined effects of trophic manipulations and cadmium addition (IV). Rock pools were chosen as “mothersystems” as they are small and isolated habitats per se and include tolerant species suitable for handling in experimental situations. An additional advantage for the ecotoxicological part of the manipulations was that D. magna is a natural component of the zooplankton community. The artificial rock pools have been evaluated by comparing patterns of seasonal development in natural and experimental rock pools (V). Experimental rock pools with phytoplankton, small-bodied zooplankton and Daphnia magna studied in 1992 and 1993 (III, IV) were used in the comparison. The food webs of the experimental rock pools included important parts of the biological processes and interactions in natural freshwater rock pools. Phytoplankton - herbivore communities with naturally co-occurring species were possible to maintain several months, i.e. exceeding more than one generation of the included species. The seasonal development resembled natural rock pools with permanent Daphnia presence and the pattern of low phytoplankton biomass in association with Duphniu presence was found in both natural and experimental rock pools and is well-known from lake studies. Most variables, with the exception of phytoplankton biomass, were in the range found in natural pools, but tended to vary less. The lower phytoplankton biomass and the lower degree of fluctuations of the variables studied were probably Partly a consequence of the absence of rain water runoff, which restricted the immigration of algal species and the input of nutrients and water from the surroundings. Occasional water inflows also dilute the 14 population of herbivores (the number of individuals L-’ decreases), which partly may decrease the grazing pressure on phytoplankton populations in natural rock pools. fish, young backswimmers prey selectively on the largest size classes of zooplankton (Scott & Murdoch 1983; Murdoch et al. 1984; III). Notonecta does not ingest its prey, but kills it by poison injected through the piercing mouth-parts after which it sucks out the prey juices (Scott & Murdoch 1983). Notonecta is an effective predator on daphnids, and capable to eliminate them under natural conditions (Murdoch et al. 1984). The scarcity of small-bodied zooplankton species in the presence of large-bodied species has been explained by size-selective predation on smallbodied zooplankton species by invertebrate predators (Hall et al. 1970; Zaret 1980; Vanni 1988; Arnott dz Vanni 1993). Another possibility, the sizeefficiency hypothesis, is that small zooplankton are competitively suppressed by large-bodied (particularly daphnids) species that are more efficient filterfeeders (Brooks & Dodson 1965; Carpenter 1988; Arnott dz Vanni 1993). The competitive ability differs between large and small zooplankton and is influenced by e.g. the amount, quality and frequency of food supply (Hall et al. 1976; Gliwicz 8z Lampert 1990). Large daphnids can ingest a larger size range of food particles than smaller cladocerans (Kerfoot 1987; Scavia and Fahnenstiel 1988; Stockner and Porter 1988; Bern 1990; Lair 199 1; Kerfoot and Kirk 1991). Consistently with the size-efficiency hypothesis, large daphnids have been shown to have lower food threshold and be competitively superior to smaller Daphnia species at low and constant food supply under predator-free conditions (Gliwicz 1990). This is given by the fact that the assimilation rate increases more rapidly with increasing body size than respiration rate. The species with the Consumer and resource regulation Experience from lake and experimental studies Fish predation has strong impact on the composition and size structure of zooplankton communities (Zaret 1980; Kerfoot & Sih 1987). Visually foraging planktivorous fish prey selectively on large zooplankton species, often resulting in a zooplankton community dominated by small species (Brooks & Dodson 1965; Zaret 1980). In fishless lakes and ponds, where invertebrate predators may be important, large zooplankton species dominate. The large size of Daphnia magna makes it vulnerable to fish predation and restricts its distribution to fishless habitats (Brooks & D o d s o n 1965; Pont et al. 1991). Although invertebrates generally are considered as less efficient predators than fish, they may also cause substantial reductions of zooplankton prey species. This has been shown for backswimmers (Notonecta), phantom midge larvae (Chaoborus) and cyclopoid calanoid and copepods (Murdoch et al. 1984; Williamson 1987; Brett 1992; Arnott & Vanni 1993; Gliwicz dz Stibor 1993). Many invertebrate predators (e.g. Chaoborus, calanoid copepods, corixids, water beetles) prefer smaller prey that are easier to handle and ingest (Swift & Federenko 1975; Williamson & Butler 1986; Williamson 1987; Black & Hairston 1988). In contrast to most invertebrates and like 15 suggested that fish, but not Chaoborus, increased the availability of phosphorus, thereby promoting phytoplankton growth. Numerous studies have shown that a combination of predation and resource limitation typically regulates freshwater plankton populations and drives seasonal successions (Threlkeld 1987; Hu & Tessier 1995; III). Data collected in freshwater ecosystems have generally shown that resource regulation, with positive relationships between prey (or resource availability) and consumer biomass, is more important at the base of the food chain (McQueen et al. 1986). Predation or top down regulation, has its main support from experimental manipulations. It appears to be most important at the top of food chains and has been manifested as negative relationships between planktivore and zooplankton biomass and between largebodied zooplankton (e.g. Daphnia) and phytoplankton biomass (McQueen et al. 1 9 8 6 ; C a r p e n t e r & Kitchell 1 9 9 2 ; Mazumder 1994; III). lowest food threshold may keep the resource level below the threshold concentration of other species sufficiently long time to cause their exclusion. The food threshold is defined as the food concentration needed to assure that assimilation equals respiration (Gliwicz 1990). The competitive outcome can be reversed if filamentous algae are present, as larger cladoceran species are more negatively affected by filaments (Gliwicz & Lampert 1990). The negative effects arise due to reductions in ingestion rates for edible algae and increases in respiration rates. The opposite view, that food threshold levels should be lower for small-bodied species and that smallbodied zooplankton should be competitively superior under low food levels due to higher foraging efficiency of smaller species on smaller particles, has also been shown (Dodson 1974; Neil1 1975; Tessier & Goulden 1987; Koivisto et al. 1992). Fish manipulations in lakes have re peatedly demonstrated the existence of trophic cascades. Commonly, increased phytoplankton biomass follows the addition or increase of planktivorous fish (see e.g. Carpenter 1988). Proposed mechanisms are reduced grazing rates, changes in the zooplankton community to a dominance of smaller species with higher nutrient regeneration rates per weight and recycling of nutrients by the planktivore (Vanni & Findlay 1990). There are few experimental studies of indirect effects of invertebrate predators on phytoplankton (III). In a mesocosm study, Vanni and Findlay (1990) found that fish and the invertebrate predator Chaoborus caused similar reductions in zooplankton biomass. However, the phytoplankton biomass increased only in the presence of fish. Their result Predation and competition in rock pool systems The effects of predation and zooplankton competition have also been examined in rock pools. Invertebrate predators dominate in rock pools and fish is normally absent. Introduction of fish in natural rock pools eliminated Daphnia magna, followed by an increase in the number of small-bodied species (Ranta et al. 1987). The number of phytoplankton cells increased in two out of three manipulations. In a study in experimental rock pools, we found that three spinedstickleback (Gasterosteus aculeafus) eliminated the D. magna population, followed by an increase in phytoplankton 16 biomass (At-n& & Koivisto personal observations). In a laboratory study, Ranta and Espo (1989) found that rock pool corixids and water beetles preyed on both chironomid larvae and D. magna. The water insects preferred chironomids and only small size classes of Daphnia wefe captured. It has also been demonstrated that corixids are able to decimate chironomids in natural rock pools (Pajunen & Salmi 1991), but it is less likely that the usually small water insect populations am able to regulate the dominating Daphnia populations in rock pools (Ranta & Espo 1989). Gammarus duebeni is omnivorous, capable to reduce D. magna populations under laboratory conditions, and is also able to feed on macroscopic green algae (Enteromotpha sp.) (personal observations). Among the herbivorous species in freshwater rock pools, the cladocerans of the genus Daphnia and Chydorus sphaericus snre generalistic filter feeders. In contrast to the size-efficiency hypothesis, it has been shown that growth and reproduction of Chydorus sphaericus at-e less affected by low food levels than is D . m a g n a (Koivisto et al. 1992). Within the genus Daphnia in rock pools, the smallest species (D. Zongispina) is competitively superior to the largest species (D. magna) (Hanski & Ranta 1983). The trophic position of the cyclopoid copepods is unclear. It has in the present studies been assumed that Cyclops sp. change from herbivory (nauplia) to carnivory (adults) during their ontogeny (Morgan 1980; Sprules 1988; Soto 1991; Adrian 1991). Cyclopoid copepodites may furthermore enter the brood pouch of daphnids and prey on eggs (Gliwicz & Stibor 1993; Gliwicz & Lampert 1994). Consumer regulation of lower trophic levels by Notonecta and Daphnia in experimental rock pools The regulation of phytoplankton and zooplankton by the backswimmer Notonecta and D. magna was studied in experimental rock pools with two and three trophic levels (III). The backswimmer was added after an initial stabilisation period of six weeks, and the experiment was terminated eight weeks later, due to high Notonecta mortality. In the two trophic level treatments, we predicted lower phytoplankton biomass in the presence of Daphnia, as compared to the treatment with small-bodied zooplankton. We also expected Notonecta to reduce Daphnia, followed by an increase in phytoplankton biomass. The result of the manipulations showed that consumer regulation was a dominant force in determining the biomass of phytoplankton and Daphnia in a planktonic rock pool community. While Daphnia reduced phytoplankton production and biomass, small-bodied zooplankton (Cyclops and Chydorus) increased with increasing phytoplankton biomass. This suggest that they were food limited and a negative correlation with Daphnia biomass hence indicates interspecific competition for limited resources. The presence of the invertebrate predator Notonecta produced a top-down effect which was similar to that reported for planktivorous fish, i. e. a reduction of Daphnia followed by an increase of small-bodied zooplankton species and phytoplankton biomass. Notonecta selectively preyed on Daphntit and the intense predation reduced the Daphnia population which was extinct within four weeks. The backswimmer seemed unable to use Chydorus or Cyclops sis food resource and starved to death after the extinction of the Duphniu population. Ultimately, however, resource availability determined the biomass at each trophic level. The food limitation of zooplankton was indicated by low egg ratios of Duphnia and a positive response of small-bodied zooplankton to increased phytoplankton biomass. The production of resource-limited Duphnia could not support Notonectu which starved to death after the extinction of Duphniu. eliminated the population within four weeks. Daphnids subjected to Notonectu predation disappeared simultaneously from control and cadmium treatments, and the cadmium x Notonectu interaction was insignificant. Conclusions Rock pool organisms are well adapted to large fluctuations in abiotic factors. This is exemplified in this thesis by a higher physiological tolerance to salinity changes in rock pool Gummarus duebeni as compared to littoral G. oceunicus. The rock pool gammarid was less affected by additions of diesel-oil and cadmium. Salinity is one of the variables that restricts the distribution of species in rock pools and high salinity negatively affected the physiological status of D. magna, leading to negative effects on growth and reproduction. The physiological index (0:N ratio) and life history variables led to comparable conclusions, supporting the relevance of the physiological index. It has also been shown that D. magna is more tolerant to e.g. copper than are other cladocerans (Koivisto et al. 1992), thus indicating that species tolerant to high degrees of natural abiotic stress are also more tolerant to pollution stress (Fisher 1977; Leblanc 1985). The relevance of a particular experimental system can be discussed on two levels: Firstly, does the model system mimic the natural system it is supposed to represent? To evaluate our experimental units it can be concluded that the seasonal development resembled natural rock pools with permanent Duphniu magna presence. The patterns of lower phytoplankton biomass in asso- Efsects of cadmium addition on trophic interactions in rock pool food webs The effects of cadmium addition were studied parallel to the above described trophic manipulations (IV). We used a control (no cadmium added) and a nominal cadmium concentration of 20 ppb. Cadmium was added at the same time as Notonectu. Cladocerans (Duphniu and Chydorus) were assumed to be the most susceptible species (Hurlbert et al. 1972; Hodson et al. 1979; Hamilton et a l . 1 9 8 8 ; Borgmann e t a l . 1 9 8 9 ; Hanazato & Yasuno 1990; Webber et al. 1992; Havens 1994), and an indirect phytoplankton, positive effect on comparable to that of Notonectu addition was expected. A significant cadmium x Notonectu interaction was also expected, i. e. a stronger reduction of Daphnia when simultaneously exposed to both mortality factors. C a d m i u m h a d a negative effect on all trophic levels, but the results did not support the hypothesis. The added cadmium strongly inhibited the phytoplankton production, which did not respond positively to Duphniu biomass. Consequently, cadmium addition and predation by Notonectu did not cause similar effects in rock pool food webs. The backswimmer was a more efficient. predator on Duphniu than expected, and 18 ciation with Daphnia presence was found in both natural and experimental rock pools and is well-known from lake studies. Most variables, with the exception of phytoplankton biomass, were in the range found in natural pools but tended to be less variable. The lower phytoplankton biomass and the lower degree of fluctuations were probably partly an effect of the absence of water inflow. However, this was an inevitable consequence of the experimental set-up. Secondly, is the “original” system ecologically representative for other ecosystems, i.e. can we extrapolate from the particular to the general? Freshwater rock pools differ in several ways from lakes: The fluctuations in physicochemical variables are larger in rock pools and probably influence the outcome of biotic interactions and the organisation of communities to a higher degree than in lakes. The food webs are simple, and planktivores are mainly represented by generalistic invertebrates unlikely to regulate the dominant daphnids. Thus, the “pelagic” part of rock pools are not similar to that of most lakes. However, rock pools may fairly well approximate other fishless habitats like ponds and bog lakes or a lake situation with low planktivore biomass due to high piscivore abundance. As shown by this thesis, tolerance to abiotic variables may also imply increased tolerance to pollutants and the use of rock pool systems may thus underestimate the impact of toxic compounds. On the other hand, if direct or indirect effects arise in this type of systems, they will probably also arise elsewhere. In my opinion, experimental ecosystems can be used to disclose potential direct and indirect impacts of e.g. addition/removal of species or environ- mental stress. The results from the experimental rock pools showed that the size-selective invertebrate predator Notonecta in low densities has the potential to produce indirect positive effects on phytoplankton and small zooplankton. This can be of importance in other fishless habitats. The addition of cadmium negatively affected all trophic levels, b u t t h e s e n s i t i v i t y differed between organisms (the studied cladocerans were more affected than Cyclops). If a corresponding change, from cladoceran to copepod dominance, occurs in natural freshwater systems, it will have major impact on phytoplankton biomass and the diet of planktivores. I suggest that the experimental rock pools systems am m o s t appropriate for studying interactions between phytoplankton and zooplankton or interactions within zooplankton. 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