Nitrogenous nutrition of the plankton in the Chesapeake Bay. 1. Nutrient availability and phytoplankton preferences

Eight stations in the main body of Chesapeake Bay and one on the continental shelf were sampled seven times over a period of 13 months to investigate the nitrogenous nutrition of the phytoplankton. The rates at which the phytoplankton were utilizing NO,, NO,, NHr+, and urea N were determined. The data demonstrate that for a large portion of the year there is inadequate N nutrient available to permit a single doubling of the particulate N. Over temperatures from 4” -28°C and salinities from 2-32&, there was a universally high phytoplankton preference for NH4+ and urea N over NO,and NO,. A relative preference index indicated that NHa’ concentrations in excess of O-5-1.0 pg-atom N liter-l almost totally suppressed NO, utilization. Urea N was used after NH,+ in order of prcfcrence, and when the sum of available NH4+ and urea N was insufficient to meet the phytoplankton N nutrient demand, NO; was used. When the sum of all available N nutrients was less than that required to satiate the phytoplankton demand, NIL’, urea N, NO,-, and NO,were all utilized at rates proportional to their availability. For the midbay region in October 1973, NO, was the dominant N nutrient present both in the water and in the diet of the phytoplankton. The Chesapeake Bay and its tributaries receive nitrogen in discharges of treated sewage from metropolitan areas of the District of Columbia and Baltimore at rates excceding 10 and 7 tonnes d-l. Carpenter et al. (1969) observed high levels of NOS(30100 pg-atoms N liter-l) in the open waters of the north half of the bay during winter and spring, levels undoubtedly sufficient to meet the nutritional needs of the indigenous phytoplankton. The persistence of such levels for several weeks in spring suggests that the rates of supply and loss were approximately balanced during this period. In summer and early autumn the NOs-concentration fell to <l lug-atom N liter-l; often it was analytically undetectable. A pronounced north-south gradient led Carpenter et al. (1969) to conclude that the Susquehanna River, which enters the bay l This research was supported by National Science Foundation grants GA 33445 and DES 7502846 and by U.S. Energy Research and Dcvelopment Administration contract E ( ll-l)-3279, document No. COO-3279-21. Chesapeake Bay Institutc Contribution 240. above the major metropolitan arcas, was the major source of NOsfor the open waters of the Chesapeake Bay, but since the distribution of NI14+ showed little spatial or temporal variability they postulated that the primary origin of NIL+ in the open water of the bay was linked to local biological activity. Studies with unialgal laboratory cultures and artificially enriched coastal seawater have shown that marinc phytoplankton prefcr NH4+ .to NOs( cf. Grant et al. 1967; Eppley et al. 1969; Eppley and Rogers 1970; McCarthy and Epplcy 1972). The preferential assimilation of NH‘*+ in the presence of NO:%has been observed or inferred from data for the Peru Current ( MacIsaac and Dugdale 1969)) the subtropical Pacific Ocean (Goering et al. 1970), the Saronikos Gulf ( MacIsaac and Dugdale 1972), and the California Current (McCarthy 1972a). The importance of urea as a source of N for natural populations of phytoplankton (e.g. Carpenter et al. 1972; Eppley et al. 1973; Webb and Haas in press) enlarges LIMNOLOGY AND OCEANOGRAPHY 996 NOVEMBER 1977, V. 22( 6) N nutrition of phytoplankton conceptualIy the importance of “regenerated nitrogen” advanced by Dugdalc and Goering (1967). Whereas NH4+ at elevated concentrations will be used in preference to urea, urea N and NOsare utilized simultaneously over a wide range of concentrations ( McCarthy and Eppley 1972). At low ambient concentrations of all forms of nitrogenous nutrient, compensation for any difference between the concentration required for optimal growth and the concentration of the most highly preferred form, i.e. NH4+, might presumably be by use of an alternate form. For NOs-, this requires an energy expenditure for both induction of the enzyme system and reducing potential (Eppley et al. 1969). In eucaryotic algae, either an ATP : urea amidolyase enzyme system or a urease system is considered essential for the assimilation of urea N (Leftley and Syrett 1973). Some marine phytoplankters do not use urea (Guillard 1963; Thomas 1968; McCarthy 1972h) and there is no evidence that either of the identified enzyme systems is other than constitutive. Low ambient lcvcls of nitrogenous nutrients in most offshore marine waters and low densities of algal biomass preclude careful scrutiny of the nutritional preferences of unperturbed phytoplankton asscmblages. The relatively high standing crop of phytoplankton and the great temporal and spatial variability of nitrogenous nutrient concentrations in the Chesapeake Bay allow us to observe in situ the rate of utilization of a single N nutrient when others are present over a wide range of concentrations. In particular, we can examine the significance of the rapidly recycled NIL+ and urea during periods of both high and low NOsconcentration. We acknowledge M. E. Loftus for technical assistance and R. W. Eppley and K. L. Webb for helpful comments on an earlier draft of this manuscript.