DYNAMICS OF THE 1990 WINTER SPRING BLOOM IN CHESAPEAKE BAY

The winter/spring 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 dif ferent 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, a nd not until mid May in North Bay. We measured biomass and nutrient co ncentrations and the rates of carbon, nitrogen, phosphorus, and silico n utilization during the development and dissipation of the bloom, and Compared ratios of these rates to the elemental ratios of the incomin g nutrients and the resulting particulate material. In North Bay, bloo m development was probably delayed due to Light limitation of carbon u ptake. Nitrogen was delivered arid utilized in excess of stoichiometri c proportions in the northern part of the Bay, eventually leading to p hosphorus and/or silicon limitation. In the mid portion of the Bay, th e mean stoichiometric proportions of the particulate nutrients were si milar to Redfield proportions, but ratios of uptake of nitrogen and ph osphorus exceeded Redfield proportions by more than 20-fold, reflectin g both the high uptake rates of nitrogen and low uptake rates of phosp horus in that region. However, only at the peak of the bloom in mid Ap ril 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(O H)(4) in South Bay were also extremely low through the bloom period, a nd thus Si(OH)4 and nitrogen, as well as PO43-, limited growth there. In addition, temperature appeared to play a key role in the collapse o f the diatom assemblage in mid May. During the early stages of the blo om in South Bay, NO3- + NO2- contributed > 60% of the total nitrogen u tilized, but by the end of the spring bloom period in May, over 50% of the nitrogen utilized was urea alone. These data underscore the need to understand how freshwater flow, ambient nutrient concentrations, te mperature, and light differ along the axis of the Bay to understand th e differential timing and magnitude of bloom development in different regions of the Bay.