Factors affecting the abundance and distribution of estuarine zooplankton, with special reference to the copepod Eurytemora affinis (Poppe).

Abstract

Factors affecting the abundance and distribution of littoral estuarine zooplankton were investigated by means of both field and laboratory studies. 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, cope pod 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. Classi~ication 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 qu~ntitatively related to temperature. Development was approximately isochronal, but the duration of the second naupliar ins tar 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 beini 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 somatic growth.