ABSTRACT: Previously reported experiments demonstrated that N addition enhanced phytoplankton growth rate more frequently and to a greater extent than did addition of P alone. Here we develop a model of nutrient-limited net phytoplankton production for the Rhode River estuary, Maryland (USA), to determine the sensitivity of observed patterns to changes in the manner in which nutrients are delivered to the system. The model successfully reproduced the seasonal pattern of phytoplankton chlorophyll variation for an average year, the relative timing of P and N limitation, and the interannual variability in the timing of the shift from P to N limitation. Manipulation of nutrient sources in the model suggested that the disappearance of N at the seaward boundary of the estuary is sufficient to produce the shift from P to N limitation. Raising the modeled N:P ratio of sediment nutrient release to a value greater than that of the phytoplankton biomass caused co-limitation by both N and P in the summer, but no manipulations produced limitation solely by P in the summer. In the model, different computed indicators of nutrient limitation (i.e. ratios of available N:P, physiological indicators of internal nutrient pools, and computed growth stimulation by nutrient additions) generally identified the same nutrient as limiting at a particular time. The model demonstrates how observed shifts in nutrient limitation result from changes in delivery of N and P from the watershed, from the sediment, and from incoming phytoplankton.

KEY WORDS: Phytoplankton · Estuary · Nutrients · Modeling

Published in MEPS Vol. 161 (1997) on December 31
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1997