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

The potential for light- and nutrient-limitation of phytoplankton prod uction was examined in the Delaware Estuary, USA, by combining a hiera rchy of experimental approaches including small-scale bioassay experim ents, ecosystem-level analysis of nutrient concentration and stoichiom etric 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-mixe d water column. However, during late spring, phosphorus (P) was found to limit growth. This observation was confirmed at each of the experim ental levels, and was related to several factors, including elevated i nput ratios (greater-than-or-equal-to 30:1) of dissolved inorganic nit rogen (DIN) to PO4 in river waters, accumulation of P into phytoplankt on, and low rates of P regeneration. During summer, P no longer limite d production. At this time DIN:PO4 ratios and bioassay experiments rev ealed the potential for nitrogen (N) limitation - particularly in the lower estuary - while particulate composition ratios and ecosystem nut rient flux estimates gave contradictory evidence. From these data it a ppears that N was potentially limiting to phytoplankton biomass but th at the constant flux of N from upstream and rapid N regeneration maint ained 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 re gulated 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.