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Abstract
Estuarine-coastal ecosystems are rich areas of the global ocean with elevated rates of organic matter production supporting major fisheries. Net and gross primary production (NPP, GPP) are essential properties of these ecosystems, characterized by high spatial, seasonal, and inter-annual variability associated with climatic effects on hydrology. Over 20 years ago, Nixon defined the trophic classification of marine ecosystems based on annual phytoplankton primary production (APPP), with categories ranging from “oligotrophic” to “hypertrophic”. Source data consisting of shipboard measurements of NPP and GPP from 1982 to 2004 for Chesapeake Bay in the mid-Atlantic region of the United States supported estimates of APPP from 300 to 500 g C m−2 yr−1, corresponding to “eutrophic” to “hypertrophic” categories. Here, we developed generalized additive models (GAM) to interpolate the limited spatio-temporal resolution of source data. Principal goals were: (1) to develop predictive models of NPP and GPP calibrated to source data (1982 to 2004); (2) to apply the models to historical (1960s, 1970s) and monitoring (1985 to 2015) data with adjustments for nutrient loadings and climatic effects; (3) to estimate APPP from model predictions of NPP; (4) to test effects of simulated reductions of phytoplankton biomass or nutrient loadings on trophic classification based on APPP. Simulated 40% decreases of euphotic-layer chl-a or TN and NO2 + NO3 loadings led to decreasing APPP sufficient to change trophic classification from “eutrophic’ to “mesotrophic” for oligohaline (OH) and polyhaline (PH) salinity zones, and from “hypertrophic” to “eutrophic” for the mesohaline (MH) salinity zone of the bay. These findings show that improved water quality is attainable with sustained reversal of nutrient over-enrichment sufficient to decrease phytoplankton biomass and APPP.
Highlights
Spatial and seasonal differences were evident in mean, monthly euphotic-layer chl-a and net primary production (NPP) based on shipboard measurements in Chesapeake Bay from 1982 to 2004 (Fig. 4a–c)
Statistical models of NPP and GPP were developed for Chesapeake Bay with adjustments for climatic effects on hydrology, calibrated with 100 s of shipboard measurements from 1982 to 2004
Model predictions of NPP based on water-quality properties in the 1960s and 1970s and 1985 to 2015 as inputs supported computations of annual phytoplankton primary production (APPP)
Summary
CB cruises (1982–83) consisted of an initial transect for horizontal mapping to determine salinity, temperature, chl-a, nutrient, and turbidity gradients and to select stations for measuring NPP. ProPhot and FITS cruises (1987–88) and LMER PROTEUS cruises (1989–94) occupied stations along a north-south transect to map water-quality properties and to measure NPP and ancillary properties. NASA cruises (1993–94) occupied three stations per cruise in the lower bay, plume, and adjacent shelf waters interspersed with mapping legs to measure NPP, GPP, and ancillary properties. LMER TIES cruises included legs for horizontal and vertical mapping, in-situ sampling of Cruise CB-1 CB-2 CB-3 CB-4 ProPhot-23 ProPhot-24 FITS-13 FITS-15 ProPhot-25 ProPhot-26 ProPhot-27 LMER 89-1 LMER 89-2 LMER 89-3 LMER 89-4 LMER 89-5 LMER 89-6 LMER 89-7 LMER 89-8 LMER 89-9 LMER 90-3 LMER 90-4 LMER 90-5 LMER 90-6 LMER 90-7 LMER 90-8 LMER 90-11 LMER 91-3 LMER 91-4 LMER 91-6 LMER 91-7 LMER 91-9 LMER 91-10 LMER 91-11 LMER 91-11 LMER 92-03 LMER 92-06 LMER 92-09 LMER 93-06
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