Physical and Biological Controls on Sediment and Nutrient Fluxes in a Temperate Lagoon

The lack of riverine inflow and shallow depths in coastal lagoons make the fluxes of nutrients and sediment across the sediment-water interface important controls on primary productivity in these systems. However, the physical-biological coupling that controls these fluxes is not fully understood. Experiments using a Gust microcosm were conducted on samples from Hog Island Bay, a shallow lagoon on the Delmarva Peninsula to determine the physical and biological controls on sediment and nutrient fluxes. Two of the dominant benthic primary producers, seagrass and macroalgae, have traditionally been considered sediment stabilizers. However, at low densities, these primary producers can increase sediment suspension by as much as 97% through flow diversion around isolated shoots or abrasion of the bed by saltating macroalgae. This increased sediment suspension during forcing events may make developing seagrass beds particularly susceptible to light limitation. These primary producers also affect erodibility of the sediment bed, with increased measured erodibility in the summer months attributed to trapping of fine material by dense macroalgal mats, as well as bioturbation by benthic fauna. While these benthic primary producers affect the physical process of sediment suspension, their growth and productivity may be affected by the interaction of physical forcing and nutrient flux. During forcing events, two mechanisms, desorption and porewater advection balanced by biological uptake, create ammonium fluxes an order of magnitude greater than those measured under low-flow conditions. However, these fluxes are likely not sustained and decrease quickly (on the order of hours). This pulsed increase in nutrient availability may create a competitive advantage for fast-growing ii species capable of surge uptake of nutrients, such as macroalgae and phytoplankton. In this sense, the timing of nutrient fluxes may be as important as the cumulative magnitude.

Note: Abstract extracted from PDF text