Download Biodiversity of Marine Sediments

Publication no. 2016 Netherlands tnstitu1eof Ecology
NIOo-CEMO, Yerseke
Biodiversity
of Marine
Sediments
C. Heip
Centre for Estuarine and Coastal Ecology,
Netherlands
Institute
of Ecology,
Vierstraat 28, 4401 EA Yersek:e , The Netherlands
Introduction
Biodiversity of the marine environment in general and of marine sediments
in particular is poorly known, both in descriptive terms of species richness and
its distribution
along latitudinal and depth gradients, and in terms of the
ecological and evolutionary processes that regulate it. In a recent, otherwise
excellent account on species diversity in ecological communities (Ricklefs and
Schluter, 1993), the marine benthos is not even mentioned and only two
chapters deal with the marine environment at all (Underwood and Petraitis,
1993; i\1cGowan and Walker, 1993). This ignorance or lack of interest is not
justified as the marine benthos occupies 70% of the Earth's surface and is an
important agent in major global biogeochemical cycles.
Overall, at our present state of knowledge, marine biodiversity appears
to be low: about 200,000 marine animal species, perhaps 20,000 marine plant
species and an even much lower number of marine viruses and microorganisms (bacteria, fungi, protozoans,
microalgae) have been scientifically
described. Of the 200,000 described animal species, only a few thousand are
planktonic,
about 130,000 are from hard substrates and about 60,000 from
sediments. The marine plant inventory is probably more complete than that
of the animals since there are no deep-sea plants. Even if the true number of
marine species was an order of magnitude lower than that of the land, on a
higher taxon level the marine environment is certainly much richer than the
land. Of the 33 animal phyla, only five do not occur in the seas and 13 are
endemic to it (Grassle et al., 1991). This implies that genetic, biochemical, and
physiological
diversity is also much higher in the oceans than on land
(Lasserre, 1992). This chapter will deal exclusively with the fauna of marine
sediments. Marine sediments cover most of the Earth's surface and although
they resemble terrestrial soils in some respects, they also have many unique
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139
Biodiversity
Table 10.1. Total number of macrofaunal
different deep-sea areas.
species on the specified
of Marine Sediments
total surface sampled in
Number of
Surface
species
sampled
Area
278-351
1.62m2
1.62 m 2
1.25 m 2
W. Atlantic,
2100 m
Grassle and Maciolek
(1992)
W. Atlantic,
1500 m
Grassle and Maciolek
(1992)
0.25 m 2
Rockall Trough, 1800-2900
NW Atlantic, 2800 m
324-363
315
146
130
0.32 m 2
Author
San Diego Trough, 1230 m
Jumars
m
(1976)
Gage (1979)
Rowe et at. (1982)
diversity. As an example, the nematode fauna from the continental slope on
the west coast of Corsica had a species richness between of 101-148 per sample
of 10cm2 at depth of 160-1000m (Soetaert et al., 1991), with many cogeneric species. For macrofauna, Grassle and Maciolek (1992) calculated that
the number of species in deep waters typically numbers many hundreds in
total sampled areas of 1-2 m 2 (Table 10.1).
Local diversity in terrestrial communities is often explained in terms
of species interactions, i.e. competition, predation. Related species living
together may coexist by partitioning resources (Lack, 1944) and niches can be
described as part of a multidimensional space that is defined by resource axes
which represent the biotic and abiotic factors along which resources are partitioned (Hutchinson, 1959). This partitioning may be part of a process leading
to increased species diversity (McArthur, 1958) because of specialization but
there is a limit to this and thus to the number of species in a community. This
is the principle of limiting similarity (McArthur and Levins, 1967) and a large
body of theory was developed in the 1970s that essentially was based on the
notion that equilibrium properties of communities, including the number of
coexisting species, were determined largely by species interactions (Schluter
and Ricklefs, 1993).
Most of the evidence for the importance of species interactions in creating
and maintaining local diversity comes from terrestrial studies. The marine
studies, mostly from intertidal environments (Connell, 1983; Reise, 1985:
Underwood and Petraitis, 1993), are all small-scale. The evidence for competitive interactions in deeper water benthos is circumstantial. One interesting
example is the spatial segregation of nematode species in the 5 em top sediment
layer of a station on the continental slope off Corsica. Six species of the genus
Sabatieria co-occur that all have the same mouth structure and therefore
probably the same food. Also six species of the genus Acantholaimus
cooccur, but they have different mouth structures and presumably different
food. The Sabatieria species potentially competing for the same food source
live at different depths in the sediment whereas the Acantholaimus
species,
that already evolved towards different morphologies,
do not (Fig. 10.1)
(Soetaert et al., 1995). The surprising consequence of such small-scale segregation is that nematode communities from the same depth layer in different
141
Biodiversity
of Marine Sediments
143
stations, tens of kilometres apart in water depths differing hundreds of metres,
are more similar than communities living at the same station in two adjacent
depth-layers one centimetre from each other.
Besides species interactions, disturbance is often invoked as a diversityregulating mechanism in terrestrial environments. Again the importance of
regular or irregular disturbances to explain patterns of diversity will be different on local and on large scales. Increased habitat heterogeneity is created
by the activity of the biota themselves. Patches consist of small-scale biogenic
structures, such as burrows, tubes, feeding pits, trails etc. On the mesoscale,
habitats are often not discrete but continuously changing due to some overriding physical factor such as wave energy, current velocity, fronts etc.
Grassle and Maciolek (1992) suggest that high overall diversity in the deep sea
is maintained by the input of small patches of ephemeral resources and the
disturbance that results from the activities of individual animals. In this
respect bottom mounds, burrows, feeding pits, other animals such as sponges
or xenophy ophoreans have been mentioned. Food input is patchy and rarer
as depth increases. Food inputs may be pulsed and tend to accumulate in
depressions and burrows made by animals on the deep-sea floor. Rare species
tend to be associated with these rare resources.
Regional and Global Biodiversity
Patterns
Trends in diversity on a truly global scale have been documented mainly from
the terrestrial environment, but why these trends exist is not really understood
(Clarke, 1992). From the marine environment, Thorson (1957) showed a pronounced increase in the species richness of epifauna from hard substrates
towards the tropics, but the number of macrofaunal species in sediments
appeared to be roughly the same for arctic, temperate and tropical areas. On
the other hand, Stehli et al, (1967) clearly demonstrated a diversity trend of
bivalve molluscs at species, genus and family level from the tropics to the poles
and in a later study (Stehli et al., 1975, cited in Clarke, 1992) demonstrated
the same for foraminiferans. Since both groups form calcareous skeletons,
Clarke (1992) suggested that perhaps such trends may not exist in other
taxa. However, recently, global-scale latitudinal trends in deep-sea epifauna
(Gastropoda, Bivalvia and the non-calcareous Isopoda) have been described
by Rex et al. (1993) from the Atlantic Ocean. All three taxa showed highly
significant latitudinal gradients in the North Atlantic with elevated tropical
diversity and depressed diversity in the Norwegian Sea.
On a somewhat smaller, regional scale, trends in benthic biodiversity have
been described from the whole North Sea (Heip et al., 1992). A clear trend
in biodiversity with latitude was found, but this trend was opposite for
macrofauna, where diversity increases toward the north, and copepods, where
diversity increases toward the south (Fig. 10.2). The higher diversity of
copepods in the south (38 species per sample) is easily explained by the
presence of sands with a median grain size larger than 200 J1-m, permitting
Biodiversity
of Marine Sediments
145
the existence of numerous small and slender interstitial (living between sand
grains) species. For the macrofauna there is a regular increase of diversity at
least between 51 and 58°N, mainly due to polychaetes, but which also exists
to a certain degree within the other three main macrofaunal groups (Fig. 10.2).
It is more difficult to explain these patterns for the macrofauna than for
copepods. Historically, the southern part of the North Sea was dry land until
4000 years ago and colonization of this area occurred both from the south and
the north. However, the deeper areas to the north are much richer in species
that would easily colonize the newly available North Sea habitats. In the
modern situation the existing current .and productivity and sedimentation
patterns are all important aspects to explain biodiversity patterns, and human
activity, especially the fisheries, is becoming a more and more dominant factor
as well.
Regional patterns in deep sea biodiversity are also most often explained
in ecological and historical terms. In ecological terms the congruity in globalscale patterns of diversity may be explained by coupling between surface and
sedimentary processes. Such coupling is probably much more intense than
anticipated even a few years ago. It has been shown that the sinking rates of
particles are much higher than previously thought and that a response between
benthic activity to accumulation on the sediments of detritus derived from
primary production at the surface is possible and indeed exists. Surface productivity increases poleward as does benthic biomass and perhaps the often
observed inverse relationship between productivity and diversity also holds
for the marine benthos. Another reason may be that the carbon flux is more
variable in the north. Historical factors may also be important. The low diversity in the Norwegian Sea has been attributed to the Quaternary glaciation
and the effects of the sea ice cover. In prosobranch gastropods local diversity
is correlated with regional diversity and the dispersal potential of the regional
species pool (Stuart and Rex, cited in Rex et al., 1993). Local diversity thus
appears to be regulated by colonization from the regional species pool and
reflects the historical evolutionary build-up of regional diversity.
How Many Species Exist In Marine Sediments?
Grassle and Maciolek (1992) published a very extensive study of deep-sea
diversity from ten stations along 176 km of the 2100 m isobath (depth contour)
off New Jersey and Delaware in the US and four additional stations at 1500 m
and 2500 m depth. In these stations a total of 798 species representing 171
families and 14 phyla were identified on a total sampled surface of 21 m 2. Of
these species, 460 (58%) were new to science. About 20% of the species were
found at all ten 2100 m stations and 34 % occurred at only one station. Of the
total soft-sediment fauna 28 % of species occurred only once and 11 % only
twice.
The number of species found rises continuously as more samples and
individuals are collected. At a single station species were added at a rate of
Biodiversity
of Marine Sediments
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