Site: Virginia Coast Reserve LTER
Eutrophication of shallow coastal bays typically causes a shift in dominance from seagrass and perennial macroalgae to ephemeral, bloom-forming algae.
P. B. Christensen.

Shallow coastal ecosystems are highly sensitive to rapid changes in population and land use occurring in the coastal zone. Nutrients associated with sewage, fertilizer, and other by-products of anthropogenic development flow from streams, rivers, and groundwater aquifers into coastal waters. As a result, the rate of nutrient loading to coastal bays is directly related to agricultural, forest, and suburban/urban land cover in adjacent mainland watersheds.

High rates of nutrient loading are a common cause of eutrophication, which can have devastating effects on coastal marine ecosystems (e.g., algal blooms, dead zones). The response of deeper, estuarine systems to nutrient loading is relatively well-understood, but VCR researchers have shown that conceptual models designed for estuaries do not apply to shallow coastal bays because of key ecological and hydrological differences.

First, primary production in shallow bays is dominated by benthic (bottom-dwelling) plants, rather than phytoplankton (which dominate in deep estuarine systems). Researchers at the VCR found that nutrient assimilation by benthic plants and sediment microbial processes are the most important factors determining nutrient retention and turnover in coastal bays; consequentially, these processes modulate the impacts of nutrient loading.

Second, shallow coastal bay waters tend to be unstratified, in contrast to deep estuaries. As a result, the benthic plant-microbe community interacts with the entire water column (rather than the bottom waters only, as in deep estuaries). This strong coupling plays a key role in controlling oxygen availability, nutrient cycling, and phytoplankton production.

If chronic eutrophication occurs, benthic plants may be replaced by ephemeral algal communities, causing nutrient cycling rates to become more erratic and decreasing the retention time of nutrients in plant biomass. These changes decelerate the removal of nutrients from the system, creating a positive feedback that reinforces continued eutrophication. Understanding the role of benthic plants and microbes in nutrient cycling is critical to predicting the response of shallow bays to nutrient loading as coastal land use changes in the future.

Graph for
In shallow coastal systems where light reaches the seafloor, benthic plants and bacteria mediate nutrient cycling and play a key role in removing or transforming nutrient inputs from watersheds before they reach the coastal ocean.
A. Hardison, 2009.