MEPS 535:29-45 (2015)  -  DOI: https://doi.org/10.3354/meps11386

ABSTRACT: Block Island Sound (BIS) and Rhode Island Sound (RIS) are adjacent inner continental shelf ecosystems with contrasting hydrographic regimes. BIS exhibits more energetic tidal mixing, and water column stratification remains weak but persists year-round due to nearby estuarine exchange flow; RIS is less influenced by estuaries, and more seasonal with strong stratification in summer. We compared annual cycles of phytoplankton biomass and primary production in BIS and RIS using measurements (surface chlorophyll, ¹⁴C primary production), primary production models (Webb/Platt and BZE models), and satellite ocean color products. During 22 mo of sampling, measured surface chlorophyll was not significantly different between BIS (mean = 1.86 mg m^-3) and RIS (1.69 mg m^-3), and bimodal peaks of phytoplankton biomass and production occurred concurrently in both Sounds. In contrast, a 12 yr ocean-color based chlorophyll time series indicated higher long-term average surface chlorophyll in the more well-mixed system (BIS, mean = 1.50 mg m^-3; RIS, mean = 0.86 mg m^-3). BIS annual primary production (318 to 329 g C m^-2 yr^-1) was higher than RIS (239 to 256 g C m^-2 yr^-1; p < 0.001). These differences were most apparent during the summer, concurrent with the largest differences in water column stratification. Phytoplankton bloom phenology was driven by physical processes, with chlorophyll significantly related to water column stratification (r = -0.51, p = 0.01), depth of the euphotic zone (r = -0.54, p = 0.05), and surface water salinity (r = 0.54, p = 0.04). Primary production was correlated with surface water temperature (r = 0.57, p = 0.03) but the mechanisms underlying production differences between the Sounds remain unresolved. We hypothesize that different hydrographies give rise to different productivity between the Sounds.

KEY WORDS: Chlorophyll a · Primary production · Continental shelf · Stratification · Remote sensing · Empirical model