Nutrient Enrichment Studies in a Coastal Plain Estuary: Phytoplankton Growth in Large-Scale, Continuous Cultures

Abstract

The response of phytoplanktonic growth to experimental nutrient addition in a tributary of the Chesapeake Bay estuary varied with season and nutrient availability. In bioassay experiments employing outdoor 0.5-m³ continuous cultures freshly inoculated with natural phytoplankton from the Patuxent River estuary, supplements of N, either as ammonium or nitrate, enhanced growth greatly during the low-flow, late-summer season — a period when N:P ratios of dissolved inorganic nutrient standing stocks are characteristically below 5:1 (by atoms). Growth response to N addition was very rapid (1 d after the start of an experiment), implying that phytoplankton in the bioassay were N limited when removed from the estuary. PO₄³⁻ addition enhanced phytoplankton growth during the late-winter, high-flow season, when N: P ratios (as previous) typically exceed 90:1 (by atoms), but the response lagged enrichment by at least 4 d and biomass levels achieved in these cultures were less than one third of those achieved in the N-enriched cultures during the late summer. The great seasonal variability in river flow, nutrient regimes, and the response of natural phytoplankton assemblages to nutrient enrichment in the Patuxent, and similar estuaries, suggest that management strategies to improve "water quality" will need to consider N as well as P additions.