Diurnal and seasonal fluctuations of organisms in a north Florida estuary

Abstract

Monthly samples of demersal fishes and invertebrates were taken in an unpolluted, river-dominated estuary in north Florida (Apalachicola Bay) for a 2-year period. Trawling methods were examined. Small (2-min) repetitive samples yielded substantially higher numbers of individuals and species than single (14-min) trawl-tows. Various methods of analysis were used to determine adequate sample size for comparative analysis of the results. Several species richness and diversity indices were compared. Regular diurnal (24-h) and seasonal variations of such parameters were related to complex interactions which included river flow, salinity variations and temperature changes. The various species diversity indices were highly correlated in spite of theoretical distinctions. There were varying relationships of such parameters with their richness and equitability components and these relationships were not always the same for fishes and invertebrates. There were basic differences in species composition and numbers of individuals of invertebrates taken throughout a 24-h period. For fishes, such variations were primarily quantitative with some short-term alteration of community structure. Seasonal peaks of numbers of individuals, numbers of species and species diversity usually occurred during summer and fall periods. Although there were some variations, a general pattern of an annual double peak of fish and invertebrate richness and diversity was noted. Nocturnal patterns were more clearly defined than diurnal ones. Relative dominance remained high, with a seasonal succession of dominant fish and invertebrate species. It was postulated that there was a constantly changing series of interactions of the various community components that precluded a single mechanism for the observed phenomena. Apalachicola Bay was seen as an unpolluted system that underwent considerable seasonal fluctuations of richness and diversity in response to extreme variations of natural (physical) functions. Such changes were stable over time, and this form of variation was seen as a considerable limitation to the general use of species diversity as an indicator of pollution and other man-induced activities in such estuarine systems.