This paper presents evidence of a seasonal shift from P to N as the nutrient limiting the accumulation of algal biomass in Chesapeake Bay. Following the winter/spring maximum in freshwater runoff, (1) the ratio of dissolved inorganic nitrogen to soluble reactive phosphorus (DIN/PO,) was greater than the N/P of algal biomass; (2) alkaline phosphatase activity was high; (3) phosphate turnover times were short; (4) ammonium turnover times were long; and (5) growth rates of phytoplankton were stimulated by additions of phosphate but not by additions of ammonium or silicate. During the period of low runoff in summer, all indicators reversed, and N limited algal growth rates. Silicate concentrations also showed evidence of biological depletion in spring, which may have limited diatom abundance. Due to the concordance of all indicators at large and small scales, we argue that phytoplankton growth rates exert primary control over biomass accumulation. We conclude that P and Si limit the accumulation of algal biomass along the major axis of Chesapeake Bay in spring, whereas N limits algal accumulation in summer, similar to the conclusions of D'Elia et al. (1986; Can. J. Fish. Aquat. Sci. 43: 397-406) for the Patuxent subestuary. Controlling eutrophication of the Bay and its subestuaries will require basin-specific management practices for both N and P reductions in influent waters. Such management efforts will provlde ecosystem tests of nutrient hmitation on a scale similar to those successfully conducted in lakes.

The winterkpring bloom of 1990 in Chesapeake Bay. USA, was prolonged and well developed, relative to other recent years, along the axis of the Bay. However, the bloom did not occur uniformly along the axis of the Bay, but rather developed and dissipated at different times in different regions of the Bay. The peak of the bloom progressed northward and was observed in late March in South Bay, early April in Mid Bay, and not until mid May in North Bay. We measured biomass and nutrient concentrations and the rates of carbon, nitrogen, phosphorus, and silicon utilization during the development and dissipation of the bloom, and compared ratios of these rates to the elemental ratios of the incoming nutrients and the resulting particulate material. In North Bay, bloom development was probably delayed due to light limitation of carbon uptake. Nitrogen was delivered and utilized in excess of stoichiometric proportions in the northern part of the Bay, eventually leading to phosphorus and/or silicon limitation. In the mid portion of the Bay, the mean stoichiometric proportions of the particulate nutrients were similar to Redfield proportions, but ratios of uptake of nitrogen and phosphorus exceeded Redfield proportions by more than 20-fold, reflecting both the high uptake rates of nitrogen and low uptake rates of phosphorus in that region. However, only at the peak of the bloom in mid April did transient phosphorus limitation of growth occur at Mid Bay. In contrast, ratios of nitrogen to phosphorus uptake rates in South Bay were considerably below Redfield proportions, primarily due to the low availability and low uptake rates of nitrogen. Concentrations of Si(OH), in South Bay were also extremely low through the bloom period, and thus Si(OH), and nitrogen, as well as pod3-, limited growth there. In addition, temperature appeared to play a key role in the collapse of the diatom assemblage in mid May. During the early stages of the bloom in South Bay, No3-+ NOzcontributed >60% of the total nitrogen utilized, but by the end of the spring bloom period in May, over 50% of the nitrogen uthzed was urea alone. These data underscore the need to understand how freshwater flow, ambient nutrient concentrations, temperature, and light dlffer along the axis of the Bay to understand the differential timing and magnitude of bloom development in different regions of the Bay.

Pigment concentrations and photosynthetic rates of phytoplankton in Chesapeake Bay were examined to determine short-and long-term responses to light. Samples were collected horizontally along a turbidity gradient and vertically relative to the depths of the pycnocline and photic zone. This approach provided phytoplankton assemblages that were exposed to light differing in both ~ntensity and time-scale of change. Photosynthesis-irradiance (P-I) curves and the parameters aB and PmB were assayed at light intensities and wavelength bands that occur in situ. Short-term (minutes to hours) changes in light elicited responses in the distribution of aB and PmB coordinate pairs along a single line. Long-term (days to weeks) differences in light elicited a change in the relation of a* to PmB, manifested as a shifted slope of the linear correlation of these parameters. Photosynthetic efficiencies were similar in light sources of narrow ('blue-green' or 'orange') and broad spectra ('white'), indicating the successful harvesting of light of these spectral qualities by photosynthetic pigments in resident phytoplankton. This similarity in photosynthetic efficiency represents a long-term response that may depend on floral composition and differences In pigmentation. Concentrations and ratios of photosynthetic pigments revealed seasonal and spatial differences in floral composition of phytoplankton in Chesapeake Bay. The presence of chrysophytes, diatoms, and dinoflagellates was indicated by low molar ratios of chl a and c during spring and summer, while the abundance of cyanobacteria (bluegreen algae) was documented by high a:b and a:c ratios in autumn. These findings have implications for the use of P-I curve parameters in diagnosing photoadaptive responses by phytoplankton and for the seasonal abundance of cyanobacteria possibly associated with eutrophication of Chesapeake Bay.