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EVPP 550 Waterscape Ecology and Management – Lecture 10 Professor R. Christian Jones Fall 2007 Phytoplankton Patterns of Abundance • Seasonal - Winter – In temperate lakes, phytoplankton are generally greatly reduced during winter due to low temperature and ice cover which impedes light transmission – However, over the winter nutrient concentrations increased due to decomposition and sediment release Phytoplankton Patterns of Abundance • Seasonal - Spring – With abundant nutrients in place, rapid growth occurs in spring when light and temperature again become favorable – In shallow lakes, increase in ambient light alone is sufficient to start the bloom – In deeper lakes, may need to get stratification before light and temperature reach their optima – In most lakes, spring bloom is dominated by diatoms Phytoplankton Patterns of Abundance • Seasonal - Spring – Spring bloom may continue for several weeks, but is eventually ends when nutrients become exhausted which for diatoms may be either P or Si – Grazing may also play a role in cropping back the large phytoplankton populations Phytoplankton Patterns of Abundance • Seasonal - Summer – In many oligotrophic and mesotrophic lakes a decline occurs in summer as nutrients become limiting – Smaller algae such as small flagellates and cyanobacteria dominate as they are better able to utilize low nutrient levels Phytoplankton Patterns of Abundance • Seasonal - Fall – In these lakes a second bloom often occurs in the fall as nutrients start to be remixed into the epilimnion – Diatoms are again often dominant, but other species can also occur – In late fall, light and temperature decline, stratification breaks down and phytoplankton populations collapse Phytoplankton Patterns of Abundance • Empirical Data – A study compiled data from many lakes and found that the bimodal pattern we just described held very well for “eutrophic” lakes (here I would use the term “mesotrophic/eutrophic” – However, oligotrophic lakes did not show as clear a seasonal pattern Phytoplankton Patterns of Abundance • SeasonalHypereutrophic Lakes – In highly productive systems (hypereutrophic) growth may continued unabated through the summer forming a single large peak in late summer – Often dominated by cyanobacteria Phytoplankton Patterns of Abundance • Interannual – Cycles are fairly predictable in a given lake – Some variability due to climatic variation including flushing – In this graph the different lines represent different diatom species in Lake Windermere, UK Zooplankton - Characteristics • Taxonomy – Protozoa • Single-celled, heterotrophic, eukaryotic • Feed on bacteria and small algae • Ciliates • Amoebae • Zooflagellates Zooplankton - Characteristics • Rotifers • Small invertebrates • Multicellular, heterotrophic, eukaryotic • Suspension feeders • Rythmically beating rotating cilia near mouth creating a feeding current, also moves organism through water • Relatively small (0.2-0.6 mm) • Generation time: ~ 1 wk Zooplankton Characteristics • Rotifers – Life History • Have both sexual and asexual (parthenogenetic) reproduction • Asexual during favorable periods • Stressful conditions induce sexual reproduction which produces “resting eggs” • Resting eggs are resistant to drying, cold, heat, etc. and can hatch when favorable conditions return Zooplankton - Characteristics • Cladocera – Small invertebrate arthropods – Multicellular, heterotrophic, eukaryotic – Use jointed appendages for swimming and feeding – “water fleas” – Very characteristic of freshwater Zooplankton - Characteristics • Cladocera – Most are herbivorous filter feeders – Filter algae from the water as they swim in a rather passive fashion – Some are raptorial predators, mainly on other cladocera – Adults range from 0.3 mm up to 3 mm except Leptodora up to 10 mm – Generation time as low as 2 weeks when asexual Zooplankton Characteristics • Cladocera – Like rotifers, have both asexual and sexual reproduction – During favorable conditions, there can be many generations of asexual reproduction (eggs that don’t need fertilizing) – When stress occurs, males are produced and sexual females, meiosis occurs to produce gametes – Male gametes fertilize eggs in brood chamber producing sexual (epphipial) eggs Zooplankton - Characteristics • Copepods – Small invertebrate arthropods – Multicellular, heterotrophic, eukaryotic – Use jointed appendages for swimming and feeding – Found in freshwater, estuaries and the ocean – Very characteristic of marine zooplankton Zooplankton - Characteristics • Copepods – Some are passive filter feeders, but most go after individual particles – Take algae and small invertebrates – Adults range from 0.5 mm to 5 mm – Calanoid & cyclopoid common in plankton Calanoid Cyclopoid Zooplankton - Characteristics • Copepods – No asexual reproduction – Fertilized egg hatches into a larva called a nauplius – Nauplius undergoes a series of molts (6) before changing into a form that looks like an adult (copepodid) – Copepodid undergoes 6 further molts before becoming an adult (sexually mature) – Males and females look similar, but males have clasper – Generation time: months to one year Zooplankton Factors Affecting Growth • Two methods have been used to measure zooplankton performance – Population growth rate (r) • N(t) = N(0) ert where r is the growth rate of the population in units of 1/time – Filtration rate • Filtration rate = volume of water cleared of particles per unit time, mL or % per unit time Zooplankton Factors Affecting Growth • Food concentrations and Temperature – Zooplankton growth often seems to be limited by food and temperature – In the study cited below, r increased with temperature at each food concentration and with food concentration at each temperature – Growth rate at the highest T and food was over 7x that at the lowest combination Zooplankton Factors Affecting Growth • Food quantity and quality – Both the quantity and quality of food are important – r = b – d (birth rate – death rate) – At the lowest food concentration, birth rate was very low and death rate quite high – As food concentration increased, birth rates increased and death rates declined strongly – The green alga Chlamydomonas supported highest birth rates and lowest death rates Zooplankton Factors Affecting Growth • Filtering rates are a function of temperature and body size • In the data shown below, larger individuals filter much more water than smaller ones • For this species, filtration rates increase to 20oC and then decline Zooplankton Factors Affecting Growth • Food concentrations and Temperature – Zooplankton growth often seems to be limited by food and temperature – In the study cited below, r increased with temperature at each food concentration and with food concentration at each temperature – Growth rate at the highest T and food was over 7x that at the lowest combination Zooplankton Patterns of Abundance and Activity • Some zooplankton populations grow in a synchronized pattern • This is particularly true in the temperate and polar areas with strong seasonality • In these areas there may be only one or two generations per year • Graph on the right shows a copepod population in a Norwegian lake which has one well synchronized cohort per year Zooplankton Patterns of Abundance and Activity • Here is a second one with two synchronized populations and a resting stage • This is most common in copepods which require sexual reproduction • In the cladocerans and rotifers, there is less synchrony generally partially due to continuous asexual reproduction under favorable conditions • It’s also harder to discern the different stages in cladocerans Zooplankton Patterns of Abundance and Activity • Other factors affect zooplankton abundance and acitivity in the field such as predation • Here is a data set which found that predation by Leptodora was a major controlling factor on Daphnia populations • Note the very high birth rate (b) in July meaning they were producing lots of eggs • But r was near 0, implying a high death rate • The period of high death rate corresponded with the maximum for the predaceous cladoceran Leptodora Zooplankton Patterns of Abundance and Activity • Predation by fish is also an important regulatory factor • It has strong effects on behavior • In a lake with fish present, a strong diel migration of zooplankton was observed with zooplankton exiting the top layers during the day, presumably to avoid fish predation • In a similar nearby lake without fish, zooplankton remained in the upper layers all day which presumably allows them to feed longer Zooplankton Patterns of Abundance and Activity • In addition to these depth patterns of avoidance, there seem to be other behaviors for avoidance of fish predation • Zooplankton cluster within macrophyte beds during the day, but venture into open water at night Zooplankton Patterns of Abundance and Activity • Presence or absence of fish in a lake has a strong effect on the species and sizes of zooplankton • An important early study looked at the size structure of lakes in Connecticut with and without anchovy • This study led to the concept of “top-down” control of food webs by which predators as opposed to food sources control biological communities Zooplankton Patterns of Abundance and Activity • While top-down control seems to regulate the types and sizes of zooplankton, the total biomass of zooplankton is strongly related to food supply • Here, we see a graph showing a positive correlation between TP vs. zooplankton • The inference is that P fuels phytoplankton growth which fuels zooplankton growth, a bottom up pattern Zooplankton Patterns of Abundance and Activity • A typical seasonal pattern of zooplankton activity involves a late spring-early summer maximum (see phytoplankton seasonal pattern earlier in lecture) • Note that all 4 groups of zooplankton can play a role during the year • The numbers attained tend to be inversely proportional to the size of individuals Zooplankton Patterns of Abundance and Activity • Zooplankton can exert heavy grazing pressure on phytoplankton and create their own “topdown” effect • Their effect varies strongly with seasonal and depth patterns in abundance Grazing/filtering rates above 50%/day would exert a major control over phytoplankton. That would imply that 50% were removed on a daily basis.