Seasonal and hydrological control of phytoplankton nutrient limitation in the lower Neuse River Estuary, North Carolina

Abstract

Nutrient limitation of phytoplankton production was assessed monthly from 1987 through 1990 in the lower Neuse River Estuary, North Carolina, USA, a well-mixed, mesotrophic system. Nutrient addition bioassays indicated that the lower estuary experienced a general state of nitrogen limitation, with especially pronounced limitation during summer months, a period of high phytoplankton productivity. Bioassays conducted during spring months showed significantly greater stimulation of algal productivity with the addition of nitrogen and phosphorus than that found with nitrogen addition alone. This CO-stimulation occurred during periods when surface-water dissolved inorganic nitrogen : dissolved inorganic phosphorus ratios were elevated above typical values of < 5. Seasonal patterns in ambient nutrient concentrations revealed nitrogen maxima associated with spring, fall, and winter runoff events, with summer minima Hydrologically driven nitrogen loading exerted a strong, yearround influence on primary production and nutrient limitation characteristics. High-flow events acted to oversaturate the upper estuarine nutrient filtering capacity, resulting in increased delivery of nitrogen to the lower estuarine environment. The phytoplankton community responded to increased flow and concomitant nutrient loadings by increasing production and b~omass levels, often very rapidly. In this regard, hydrologic factors influencing nitrogen loading (terrigenous runoff, point source inputs, and wet and dry atmospheric deposition) are key determinants of the trophic state of this estuary.

Ecosystem responses to internal and watershed organic matter loading:consequences for hypoxia in the eutrophying Neuse River Estuary, North Carolina, USA

“…Previous nutrient addition bioassays (Paerl 1987, Rudek et al 1991, Paerl et al 1995 and nutrient uptake kinetic studies (Boyer et al 1994) have shown Nlimited conditions throughout this estuary. Parallel, independent bioassay studies conducted during this study period likewise showed N limitation (Paerl et al 1995, Pinckney et al 1997, with N enrichment achieving similar phytoplankton growth yields as N +P.…”

Highs and lows: The effect of differently sized freshwater inflows on estuarine carbon, nitrogen, phosphorus, bacteria and chlorophyll a dynamics

“…During the study period the only factor that helped to explain chlorophyll a concentrations from those tested was temperature, which is regularly found in estuaries (Eppley, 1972). However, it is relatively common for chlorophyll a to be also linked to nutrient availability (Rudek et al, 1991). One possible explanation for why chlorophyll a was not related to nitrogen or phosphorus may be that bacteria regularly outcompeted phytoplankton for available nutrients.…”

Anthropogenic and climatic influences on the eutrophication of large estuarine ecosystems

“…At upstream freshwater locations, P is often the growth-limiting macronutrient (Boynton et al 1982;Larsson et al 1985). At the freshwater-saltwater transition zone, P and N may both be colimiting, while the downstream mesohaline, polyhaline zones are usually N limited Rudek et al 1991;Elmgren and Larsson 2001). The spatial and temporal overlaps of N and P limitation vary widely, and are closely controlled by hydrology, morphometry, geography, and climate.…”