Long-term variations in primary production in a eutrophic sub-estuary. I. Seasonal and spatial patterns

Abstract

Daily rates of phytoplankton primary production were calculated from measurements of light saturation curves of photosynthesis for 20 yr at 6 stations on the Rhode River, Maryland (USA). Daily production, corrected for the geometry and spectrum of the underwater light field, av- eraged 1319 (range 1.4 to 15 800) mg C m −2 d −1 . Log-transformation of the exact solution for depth- integrated daily production permitted linear analysis of seasonal and spatial patterns in production and the factors that determine it. The seasonal signal was the greatest source of variation, followed by spatial then interannual. The seasonal pattern was driven by coinciding summer maxima in both the chlorophyll a (chl a) biomass, B, and the light saturated photosynthetic rate normalized to chl a, P B max. The spatial pattern was characterized by a region in which production was relatively constant despite declining depth, a station at which production was reduced by truncation of the depth profile of production, and an area where mean production was lowest but variance was highest, due to local flow causing either localized blooms or washout of biomass and high turbidity at the station furthest up the estuary. Analysis of the components contributing to the variance in production indicated that variance in B and P B max added nearly equally to it. Covariance between B and the light attenuation coefficient reduced the variance in production. The analytical approach adopted here allowed these patterns to be discerned against a high degree of overall variability, and should be similarly useful in a wide range of systems.