Factors affecting the abundance and distribution of estuarine zooplankton, with special reference to the copepod Eurytemora affinis (Poppe).
In the field, a 12-month survey was conducted at 6 stations
spanning a wide salinity range in the Forth estuary, to
investigate the influence of geographical, seasonal, tidal and
physical environmental variables on community structure. Pump
samples, in two net fractions (69 urn and 250 um) were collected
on spring and neap tides, at high and low water over 9 complete
or partial lunar cycles.
The use of two concentric nets of differing mesh size extended
the size range of specimens caught, and permitted the observation
and enumeration of small plankters such as rotifers, copepod nauplii
and early polychaete larvae. In early 1982, a clear temporal
succession of rotifers> freshwater crustacea> Haranzelleria larvae>
Eurytemora was observed.
The data acquired on field distribution and abundance were analysed
in a variety of ways. The most effective approach was found to be a
combination of polythetic, divisive classfication (Twinspan) of
sepcies data, followed by Multiple Discriminant Analysis (MDA) of
the classification using geographical, seasonal, tidal and environmental
data as the discriminating variables. The effects of geographical,
seasonal and tidal variation were removed by analysing subsets of the
data restricted to one state of a variable at a time; comparison
of these restricted analyses with those performed on larger
data sets revealed, however, that the relative influence of variables
on community structure could be readily discerned even when all
variables were considered together. Salinity and geographical
position of station were clearly the dominant factors in explaining
the species associations defined by classification analysis; organic
suspended particulate material was closely associated with these, and
temperature also but to a lesser degree. The influences of season and
primary production were linked, and were orthogonal to the influence
of the dominant variables.
Classification analysis identified three main assemblages: the freshwater
community; a low-salinity group comprising Eurytemora affinis
and Neomysis integer; A neritic assemblage dominated by Acartia spp.,
Pseudocalanus and Oithona but also including Temora, Centropages and
meroplanktonic larvae. Pseudocalanus and Oithona were more persistent
than the other neritic taxa, and were more often found in samples of
lower salinity and in the autumn and winter.
Predation and development rate are two biological factors which
directly influence the abundance and distribution of individual taxa.
In the laboratory, studies were conducted a) on the rate of predation
of Neomysis on Eurytemora and b) the effects of temperature and food
availability on the development rate of Eurytemora.
Predation rates of adult mysids on adult Eurytemora were estimated to
range up to 170 prey/day at 500 prey/litre, and the functional response
was adequately modelled by a Type II curve. It was experimentally
, demonstrated that predation rates were not reduced in the dark or in the
presence of detritus, and it is inferred from this that Neomysis
relies on random foraging rather than on visual predation. Estimated
predation rates were sufficiently high to suggest that Neomysis
predation may, at some times of the year, have a significant effect on
Eurytemora population size.
Development rates in Eurytemora were not affected by food level, but
were quantitatively related to temperature. Development was approximately
isochronal, but the duration of the second naupliar instar was consistently
longer than that of other instars, especially at lower temperatures.
Total estimated development times ranged from 39 days at 8 deg.C to
15.25 days at 20 deg.C, with the effect of temperature being more marked
at low temperatures than at high temperatures.
The results of the development study were applied to field observations
of instar body lengths, in order to estimate daily length increment for
9 dates in 1982. Field observations had indicated that, in contrast to
many other studies, body size did not bear a simple inverse relationship
to water temperature; whilst the smallest animals were observed
during the spring bloom .and midsummer, the largest specimens were collected
in September when water temperatures were still high.
Highest growth rates were estimated for August (small animals) and
September (large animals) ; winter animals, although similar in size to
September specimens, had low estimated growth rates. The large size of specimens encountered in September suggests, when considered in
conjunction with the low abundance at that time, that a switch may
have occurred from investment in reproduction to an investment in