SEASONAL PROGRESSION OF FACTORS LIMITING PHYTOPLANKTON PIGMENT BIOMASS IN THE RHODE RIVER ESTUARY, MARYLAND (USA) - II - MODELING N VERSUS P LIMITATION

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 (US A), to determine the sensitivity of observed patterns to changes in th e manner in which nutrients are delivered to the system. The model suc cessfully reproduced the seasonal pattern of phytoplankton chlorophyll variation for an average year, the relative timing of P and N limitat ion, and the interannual variability in the timing of the shift from P to N limitation. Manipulation of nutrient sources in the model sugges ted that the disappearance of N at the seaward boundary of the estuary is sufficient to produce the shift from P to N Limitation. Raising th e modeled N:P ratio of sediment nutrient release to a value greater th an that of the phytoplankton biomass caused co-limitation by both N an d P in the summer, but no manipulations produced limitation solely by P in the summer. In the model, different computed indicators of nutrie nt limitation (i.e. ratios of available N:P, physiological indicators of internal nutrient pools, and computed growth stimulation by nutrien t additions) generally identified the same nutrient as limiting at a p articular time. The model demonstrates how observed shifts in nutrient limitation result from changes in delivery of N and P from the waters hed, from the sediment, and from incoming phytoplankton.