Ecological Responses to Eutrophication and Restoration in a Shallow Subtropical Estuary
Abstract
Like many other estuaries in the world, the Indian River Lagoon (IRL) is
experiencing serious environmental health challenges in recent years. Ecosystem
challenges include the connected phenomena of Harmful Algal Blooms (HABs)
due to eutrophication and accumulation of fine-grained organic-rich sediments on
the benthos. Fine-grained organic-rich sediments, referred to as “muck”, is a source
of nutrients and fuels algal blooms in the water column, and excess particles
associated with algal blooms sink to bottom and contribute to muck. This
dissertation aims to better understand these problems by addressing two main
objectives: first, to determine whether key herbivorous mesozooplankton have the
capacity to exert top-down control on the HAB species, and second, whether to
explore alternative approaches of muck removal and ecosystem restoration to
disrupt the vicious cycle of eutrophication perpetuated by organic sediments. For
the first objective, grazing experiments revealed that Parvocalanus crassirostris,
the dominant herbivorous copepod in the IRL, exerted some top-down control on
the HAB species under laboratory conditions (Chapter 1). Additional laboratory
grazing experiments on cell size and mucus effects were examined to better
understand the low grazing rates (Chapter 2). For the second objective, estuarine ecosystems were monitored throughout a pilot aeration experiment to evaluate the
feasibility and effectiveness of aeration for muck mitigation (Chapter 3). It was
found that aeration facilitated benthic animal life, but only in the cold period when
bottom waters held the most dissolved oxygen. Planktonic communities, on the
other hand, displayed no significant responses to aeration.