Seasonal pulses of turbidity and their relations to eelgrass ( Zostera marina L.) survival in an estuary

Abstract

The light environment of one Chesapeake Bay tributary where seagrasses have decreased in abundance was described using both continuous and discrete measures of irradiance and related to the growth and survival of transplanted eelgrass ( Zostera marina L.). After 8 months of continuous growth at an upriver site, a decline and eventual complete loss of eelgrass transplants began during a month long (May–June) period of increased turbidity ( K d>3.0). Transplant loss continued even after light conditions improved ( K d<2.0). At a downriver site where there has been some natural seagrass regrowth, the pulse of high turbidity was not as evident and transplants survived. Other than this spring period of high turbidity at the upriver site, the light environments of the two areas were similar with minimum turbidity in January and maximum in the spring and summer. Annual median daily attenuation coefficients ( K d) at the upriver and downriver sites were 1.77 and 1.96, respectively, and were not significantly different ( P=0.49). Total downwelling quantum flux at transplant depths of 0.8 m below mean sea level were 2618 and 2556 mol·m −2·yr −1, or approximately 24.9 and 24.3% of annual solar PAR. The high spring turbidity pulse corresponded to an increase in non-chlorophyll particulate matter. Chlorophyll specific attenuation ( K c) accounted for 6.7–9.0% of K d in June. Differences in attenuation were greatest in the 400–500 nm spectral region. Therefore, measures of total PAR attenuation can overestimate the usable irradiance available to the macrophytes. Scalar quantum fluxes during the period of elevated turbidity were 2.7 and 13.4 mols·m −2·day −1 at the upriver and downriver sites. The duration and intensity of total PAR measured upriver during this period were insufficient to support eelgrass growth and survival, and below literature estimates for eelgrass community light compensation at in situ temperatures (20–25°C). Therefore late spring, month-long pulses in turbidity, such as measured here can account for the loss of transplanted vegetation and, potentially, explain lack of successful recruitment into formerly vegetated upriver sites.