Document Type

Article

Date of Original Version

1997

Department

Oceanography

Abstract

During 1992-1994, we made shipboard incubations suitable for determining rates of primary production in water from Boston Harbor, Massachusetts Bay, and Cape Cod Bay (Massachusetts, USA). These measurements were part of an extensive baseline monitoring program to characterize water quality prior to diversion of effluent from Boston Harbor directly into Massachusetts Bay via a submarine outfall diffuser. Production (P) was measured using whole-water samples exposed to irradiance (I) levels from ~5 to 2000 µE m-2 s-1. P-I incubations were performed on 6 surveys a year, spaced to capture principal features of the annual production cycle. The number of stations and depths examined varied between years. There were 10 stations and 2 depths sampled in 1992-1993. In 1994, we performed in-depth studies at 2 stations (Boston Harbor's edge and western Massachusetts Bay) by sampling 4 depths. Using depth-intensive 1994 data a simple empirical regression model, using information on chlorophyll biomass, incident daily light, and the depth of the photic zone, predicted integrated primary production rates derived from P-I incubations. The regression model was virtually the same as described for other coastal waters, giving confidence in general use of the model as an extrapolation tool. Using the 1994-based empirical model, we obtained favorable comparisons with production rates modeled from 1992-1993 P-I incubations. Combining the regression model with data on chlorophyll, light, and the photic zone collected on frequent hydrographic surveys (up to 16 yr-1), annual primary production was estimated for 1992-1994. Primary production in an intensively studied region of western Massachusetts Bay (21 hydrographic profile stations in an area ~100 km2) ranged from 386 to 468 g C m-2 yr-1. For a station at the edge of Boston Harbor near Deer Island extrapolations suggested production rates of 263 to 546 g C m-2 yr-1. Based on 2 stations in central Cape Cod Bay (1992-1993 only), model extrapolations suggested an annual production of 527 to 613 g C m-2 yr-1. Analyses using incubation and modeling results suggested that production variability was strongly related to fluctuations in incident irradiance, especially at daily to seasonal time scales. Chlorophyll variability secondarily influenced production, especially at seasonal to annual time scales. Finally, we provide a case where equivalent production was achieved in environments with contrasting water quality (nutrient and chlorophyll concentrations) because of variations in the depth of the photic zone (controlled by both chlorophyll and non-chlorophyll turbidity). Comparative analyses showed that our study estimates of primary production were consistent with the literature on nutrient-rich shelf environments. In conclusion, our study validated an empirical modeling approach to determining primary production in coastal marine waters.

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