Spatial and temporal variation of the phytoplankton in Cayuga Lake

The limnology of Cayuga Lake was studied from April 1972 to August 1973. The objectives were to: a) describe variation in nutrient parameters in vertical and horizontal space, within years, and historically where data were available, b) describe the phytoplankton (pigment, biomass, and species composition) along the same dimensions, c) analyze the structural patterns in the phytoplankton communities relative to chronological and spatial gradients and relate these structural patterns to theoretical models of communities, d) assess the historical changes in phytoplankton patterns and composition. Physical, chemical, and biological data were collected at approximately weekly intervals from late spring to late fall and at monthly intervals during winter at five stations distributed along the north-south axis of the lake. The parameters measured were water temperature, Secchi disc transparency, dissolved oxygen, pH, total alkalinity, suspended solids, soluble reactive silicon, nitrate-nitrogen, soluble reactive phosphorus, chlorophyll a, phaeophytin, in vivo fluorescence, and phytoplankton biomass and species composition. The phytoplankton were described according to taxonomic, size, and morphological groups. Species associations were delineated via principal components analysis. Nutrients declined from maximum values in spring to minima in mid-summer. Soluble reactive phosphorus hovered at barely detectable levels most of the summer, Other nutrients did not remain near their minimum values for long. Nitrate-nitrogen and soluble reactive silicon had slightly higher concentrations at the northern stations in spring and the reverse in summer. Soluble reactive phosphorus was generally higher at southern stations. Total chlorophyll a was highest in late spring during the diatom pulse. Southern stations had significantly higher concentrations than northern stations, Spatial differences were greatest during the diatom bloom and smallest during the bloom of Anabaena. Total chlorophyll a was often 2-3 times higher in 1972 than 1973 as a result of Tropical Storm Agnes. In vivo fluorescence was not well correlated with total chlorophyll a, Two significantly different regressions of in vivo fluorescence on total chlorophyll a were observed: one for spring and one for summer and fall, The low correlation could not satisfactorily be attributed to the reasons given in the literature for similar difficulties. Continuous profiles of in vivo fluorescence along the long axis of the lake reflected patterns observed for total chlorophyll a at discrete stations. Phytoplankton biomass was dominated at all times by cryptophytes. Diatoms were prominent in spring, chlorophytes and cyanophytes in summer. Biomass was greatest during late spring. The number of phytoplankton species was greatest in August and September. Principal component analysis of species' biomasses isolated nine communities of species. Eight of these were arrayed along a chronological gradient, one isolated a community of ubiquitous surface species and another community of species found below the thermocline. Species associated with Anabaena were predominantly motile as were the ubiquitous surface species. Communities occurring from mid- to late summer exhibited a change from relatively large cell size to small cell sizes. By using Euclidean distance as a measure of the difference between stations, it was shown that changes in between-station differences paralleled changes in total phytoplankton biomass. Changes in species composition between stations and between sampling dates seemed to precede or coincide with parallel changes in the number of species. It was proposed that the number of species was determined by the frequency and amplitude of environmental variation alternating with environmental stability where the effect of the variability on the phytoplankton is a parabolic function, i.e. high and low variability reduces the number of species. Species turnover times must be short relative to the scale of variability and competition coefficients sufficiently small that extinction does not occur in the period of stability. Changes in relative turnover time were seen as a mechanism whereby the phytoplankton could adapt to a regime of environmental variability. Historical comparisons (1910-1973) of the phytoplankton were made by converting cell count data to biomass, determining the size selectivity of earlier surveys, and comparing them with the appropriate size groups from 1972-73. Year to year variation was found to be as great as the differences between the biomass for the earlier surveys and that for 1972-73. The greatest historical change involved a greater probability of a sizeable spring diatom pulse in recent surveys. A high correlation between the maximum spring phytoplankton biomass and the average summer biomass was demonstrated. Cyanophyte biomass seems to have increased little; the biomass of troublesome cyanophytes such as Anabaena and Microcystis was variable but does not seem to have increased. An apparent dramatic increase in total chlorophyll a between 1966 and 1972-73 when viewed from the perspective of parallel changes in biomass for that period and the longer span of biomass information is just as likely part of a random or cyclic variation