TEMPORAL ALTERNATION BETWEEN LIGHT-LIMITATION AND NUTRIENT-LIMITATION OF PHYTOPLANKTON PRODUCTION IN A COASTAL-PLAIN ESTUARY

The potential for light- and nutrient-limitation of phytoplankton production was examined in the Delaware Estuary, USA, by combining a hierarchy of experimental approaches including small-scale bioassay experiments, ecosystem-level analysis of nutrient concentration and stoichiometric ratios, and light-limitation modeling. Light was found to be the predominate regulator of phytoplankton growth throughout the estuary during the winter period as a result of high turbidity and a well-mixed water column. However, during late spring, phosphorus (P) was found to limit growth. This observation was confirmed at each of the experimental levels, and was related to several factors, including elevated input ratios (greater-than-or-equal-to 30:1) of dissolved inorganic nitrogen (DIN) to PO4 in river waters, accumulation of P into phytoplankton, and low rates of P regeneration. During summer, P no longer limited production. At this time DIN:PO4 ratios and bioassay experiments revealed the potential for nitrogen (N) limitation - particularly in the lower estuary - while particulate composition ratios and ecosystem nutrient flux estimates gave contradictory evidence. From these data it appears that N was potentially limiting to phytoplankton biomass but that the constant flux of N from upstream and rapid N regeneration maintained non-nutrient-limited steady-state growth. These data document a pattern of recurring system-wide variations in the factors that limit phytoplankton production over several annual sequences. These temporal and spatial variations are related to both light availability - as regulated by incident light, suspended sediment concentration, and depth of the surface mixed-layer-and nutrient availability - as determined by riverine inputs and in situ biogeochemical processes.