Regulation of estuarine primary production by watershed rainfall and river flow

Abstract

Enhanced phytoplankton production and algal blooms, symptoms of eutrophication, are frequently caused by elevated nutrient loading, usually a s nitrogen, to coastal waters. This nitrogen is derived primarily from anthropogenic sources (urban, industrial, and agricultural) but is delivered to coastal waters through meteorological and hydrological means. We utilized a 4 yr monthly data set to investigate the effect of these upstream physical forces upon primary productivity of the Neuse River Estuary (North Carolina. USA), a large temperate coastal plain estuary Our results indicate that the magnitude of estuarine primary production and the periodicity of algal blooms can be directly related to variations in upper watershed rainfall and its subsequent regulation of downstream nver flow Future changes in preclpitatlon patterns for coastal regions may thus lead to substantlal alterations in coastal primary productiv~ty rates and patterns. Biological productivity in estuarine waters is influenced by a host of factors including latitude, season, irradiance, temperature, flow, nutrient loading and recycling, grazing, and watershed geomorphology and development (Mann 1982, Nixon 1986, Day et al. 1989). Upstream river flow has been recognized as an important influence on the primary productivity, salinity, nutrient loading, light attenuation, dissolved oxygen, and stratification of estuarine waters (Cooper & Copeland 1973, Jaworski 1981, Sellner 1987, Randall & Day 1987, Malone et al. 1988, Christian et al. 1991, Jordan et al. 1991, Rudek et al. 1991, Gallegos et al. 1992). Factors controlling river flow should, therefore, be expected to affect estuarine primary productivity. Present addresses: 'Department of Biological Sciences, University of North Carolina at Wilmington, Wilmington, North Carolina 28403, USA ' 'USGS MS 469,345 Middlefield Rd. Men10 Park, California 94025, USA ' 'Department of Horticulture. 1575 Linden Dr., University of Wisconsin, Madison. Wisconsin 53106. USA The Neuse River Estuary, North Carolina, USA, typifies coastal plain estuaries with its large drainage basin (16000 km2) , relatively shal1o.w water, and elevated nutrient loading from urban, industrial, and agricultural sources (Stanley 1988, Christian et al. 1991). Bioassays have indicated that primary productivity is predominantly nitrogen-limited in this system (Rudek et al. 1991). The phytoplankton conlmunity is dominated by centric diatoms, dinoflagellates, and cryptomonads, with community structure influenced by salinity; increased freshwater inputs favor cryptomonads, whereas diatoms tend to dominate under more saline conditions, and dinoflagellates are usually subdominant except for periodic blooms (Mallin et al. 1991). Previous research indicated that during 1988-1989 primary productivity was strongly correlated with surface nitrate concentrations and negatively correlated with salinity (Mallin et al. 1991). Also, periods of increased river flow were associated with increases in nitrate and decreases in salinity (Rudek et al. 1991). Based on this information, our objective in the present research was to examine how the interplay of physical and chemical variables affected estuarine primary productivity from a n ecosystem perspective. Accordingly, we utilized a long-term data set to investigate the extent to which river flow and its primary driving force, watershed rainfall, influenced the primary productivity of this large temperate estuary. Materials and methods. We sampled for phytoplankton productivity, taxa composition and abundance, chlorophyll a, nutrients, and physical factors at Channel Marker 6, a mesohaline station located about 10 km from the mouth of the estuary where it joins Pamlico Sound (Fig. 1). Samples were collected approximately monthly over a 4 yr perlod (1988 to 1992). Primary productivity was assessed by the 14C 0 Inter-Research 1993 200 Mar. Ecol. Prog. Ser. 93: 199-203, 1993