Impacts of Coastal Eutrophication on Benthic Ecosystem Functions

Estuaries are among the most productive marine ecosystems and macroalgae is recognized to be the most abundant and productive among the benthic marine primary producers. When eutrophic conditions are present, one of the most evident responses of this condition is the proliferation of macroalgae blooms mainly caused by opportunistic macroalgae. Nonetheless, the impacts of macroalgae blooms on sediment biogeochemistry and organic carbon storage is not fully understood, especially on unvegetated coastal areas.

The present thesis evaluates historical records of macroalgae blooms in the UK estuaries and results from field measurements of sediment biogeochemistry are presented to assess the impacts of macroalgae blooms on benthic ecosystem services by comparing impacted sites and the influence of seasonality. Furthermore, an evaluation was made of the potential for macroalgae derived carbon identification within estuarine and coastal sediments.

This work presents the impacts of excessive macroalgae blooms on sediment biogeochemistry and analyses the historical data from southern UK estuaries subject to different impacts of eutrophication and macroalgae growth. Results from sediment and porewaters data indicate the enhanced capacity for organic carbon storage within more impacted sites by eutrophication and macroalgae. Furthermore, results from sediment biogeochemistry on different seasons also suggest more accumulation of organic C during colder months, which is suspected to be due to less degradation happening. In addition, evidence of macroalgae derived carbon within sediment carbon stocks is provided. This research demonstrates the relevance of estuarine unvegetated sediments impacted with macroalgae blooms for organic carbon cycling and storage and provides basis for future exploration on macroalgae derived organic carbon in sediment carbon stocks.