Plankton patchiness and the associated distribution and growth of larval anchovies: An estuarine landscape perspective
Spatial and temporal environmental heterogeneity are defining characteristics of estuaries. Spatial heterogeneity of prey can affect the feeding efficiency of larval fishes through inconsistent rates of encounter resulting in variable growth, predator avoidance and potential recruitment success. Objectives of this study were to assess (1) spatial and temporal variability in environmental variables, densities and patch sizes of bay anchovy ( Anchoa mitchilli ) eggs (BAE) and larvae (BAL), macroand microzooplankton and (2) relationship of prey density and length-at-age of BAL within Mississippi Sound. Monthly samples were taken from May to August 1998 along two permanent transects, one onshore (∼2km S of the mainland) and one offshore (∼2km N of a near-shore barrier island), consisting of 50 stations each, spaced ∼40m apart. Environmental parameters, as well as densities of BAE, BAL, copepod nauplii and copepodites, mollusc veligers, tintinnids, hydromedusae and chaetognaths were found to be highly variable among 2 months and locations. Correlation analysis indicated that BAE abundance was positively correlated with chl a and copepod nauplii at the largest spatial scale (Pearson's r, p < 0.05), suggesting that adult anchovy may "place" BAE within prey patches. BAL abundance was consistently correlated with prey items and chl a (p < 0.05). Indices of dispersion (variance/mean ratio, Morisita's and standardized Morisita's index) and patch size estimates (Two-Term-Local-Quadrat-Variance technique) for BAE, BAL and prey indicated that all taxa were contagiously distributed with highly variable patch sizes. Analysis-of-covariance showed there was no difference in the slopes of length-at-age regressions of BAL within high and low density patches of copepod nauplii from May transects. However, BAL were larger in low prey densities than BAL collected in high prey densities during July. Landscape-scale environmental conditions within Mississippi Sound were highly variable during the short duration of this study and suggest that the spatial heterogeneity of both BAL and prey from July collections may be responsible for significant differences in length of BAL, variability which could also affect bay anchovy recruitment success. These findings also illustrate that the trophic dynamics of predator/prey are highly scale-dependent requiring that both trophic levels be sampled simultaneously in order to examine their interaction in the field.