Modelling phytoplankton deposition to Chesapeake Bay sediments during winter–spring: interannual variability in relation to river flow

Abstract

The often-rapid deposition of phytoplankton to sediments at the end of the spring phytoplankton bloom is an important component of benthic–pelagic coupling in temperate and high latitude estuaries and other aquatic systems. However, quantifying the flux is difficult, particularly in spatially heterogeneous environments. Surficial sediment chlorophyll- a, which can be measured quickly at many locations, has been used effectively by previous studies as an indicator of phytoplankton deposition to estuarine sediments. In this study, surficial sediment chlorophyll- a was quantified in late spring at 20–50 locations throughout Chesapeake Bay for 8 years (1993–2000). A model was developed to estimate chlorophyll- a deposition to sediments using these measurements, while accounting for chlorophyll- a degradation during the time between deposition and sampling. Carbon flux was derived from these estimates via C:chl- a = 75. Bay-wide, the accumulation of chlorophyll- a on sediments by late spring averaged 171 mg m −2, from which the chlorophyll- a and carbon sinking fluxes, respectively, were estimated to be 353 mg m −2 and 26.5 gC m −2. These deposition estimates were ∼50% of estimates based on a sediment trap study in the mid-Bay. During 1993–2000, the highest average chlorophyll- a flux was in the mid-Bay (248 mg m −2), while the lowest was in the lower Bay (191 mg m −2). Winter–spring average river flow was positively correlated with phytoplankton biomass in the lower Bay water column, while phytoplankton biomass in that same region of the Bay was correlated with increased chlorophyll- a deposition to sediments. Responses in other regions of the Bay were less clear and suggested that the concept that nutrient enrichment in high flow years leads to greater phytoplankton deposition to sediments may be an oversimplification. A comparison of the carbon flux associated with the deposition of the spring bloom with annual benthic carbon budgets indicated that the spring bloom did not contribute a disproportionately large fraction of annual carbon inputs to Chesapeake Bay sediments. Regional patterns in chlorophyll- a deposition did not correspond with the strong regional patterns that have been found for plankton net community metabolism during spring.