Nutrients and sediments are both a blessing and a curse in the world’s coastal bays and shallow seas. In just the right amount, nutrients, and particularly nitrates, fuel the growth microscopic algae that feed young fish and shellfish, driving the diverse food-chains and, ultimately, feeding people and supporting coastal communities. Similarly, the right amount of sediments provide critical minerals and the very substrate that allows marshes and mangroves to take hold and flourish, to grow upwards or seaward in an ongoing give-and-take with the sea as its level changes.
By contrast, excessive nutrients and sediments, arriving from eroding soils, agricultural chemicals, runoff from livestock and from untreated sewage, are a curse that can lead to eutrophication—an overabundance of algae. At first algal blooms simply cloud the water, preventing sunlight from reaching the seabed where seagrass or seaweeds might otherwise grow. But the decomposition of dying algae sucks the oxygen out of the water. When this happens in excess, the water becomes “hypoxic,” with so little oxygen that fish and other animals must escape or suffocate. These places are dead zones, and there are growing numbers of them around the world.
The balance is critical and as it turns out, maintaining the right balance is a function of controlling what comes in to our coastal waters, as well as what is scrubbed out naturally by “ecosystem engineers,”— those plants and animals found in coastal waters that modify their surrounding environment through their structure or biology. Mangroves, marshes, sponge “gardens” and bivalve reefs are all good examples of coastal engineers that buffering against nitrogen and sediment imbalances every day.
Oyster reefs and other bivalve aggregations are among the most effective of all the cleaners of coastal waters, filtering vast amounts of water from which they collect and digest microscopic algae for food. A single oyster can filter 180 liters of water every day. Great banks of oysters can thus yield measurably clearer waters, supporting underwater grasses and other plants that need light to survive underwater. These plants, in turn, yield additional benefits, like fish production and carbon storage, completing something of a virtuous cycle. As they filter water and feed, oysters also deposit waste material onto surrounding sediments where it fuels growth of helpful bacteria that digest this nitrate-rich waste. In doing so, these bacteria release nitrogen back to the atmosphere as harmless gas (78 percent of the air we breathe is inert nitrogen gas) through a process called “denitrification.” It is no wonder, then, that between their filtration and removal of nitrogen pollution, oyster reefs are becoming recognized as a critically important part of our estuaries. Scientist call oysters a “keystone species” in recognition of their important role in estuaries.
Unfortunately, in our quest to harvest these delectable bivalves around the world, and in our haste to develop the lands and waters they call home, oyster reefs have become the most imperiled marine ecosystem on Earth, with an estimated 85 percent loss globally.
Mapping Ocean Wealth: Oyster Restoration for Water Quality
In our efforts to better quantify the benefits provided by oysters, and to further motivate both conservation and restoration, MOW scientists have built unique models of oysters and water filtration. We need to know ‘how much is enough?’ to make an appreciable difference to the water quality of a bay. One approach for answering that question is to have enough oysters to filter the volume of water in a bay in about the same time it takes the tide and river flow to replace this water over time. This is known as the “residence time” of water in an estuary. At this level of abundance and filtration, oysters are grazing and feeding at a rate that is probably sufficient to clear the water and keep algae blooms in check for an entire bay.
To understand the variability in the extent of filtration by oysters in the bays and estuaries of the USA, we constructed a mathematical model that estimates filtration based on individual oyster size, abundance and reef area, as well as water temperature. Using knowledge of the past and present extent and condition of oyster reefs in estuaries around the USA, together with ecological understanding of their filtering capacity we have been able to model the volume of water being filtered per bay. These are expressed as a proportion of the total water passing through the bay. Historically, many bays could filter several times over the volume of water passing through them now.
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Mangroves and saltmarshes play a critical role in slowing the water flows and allowing sediments to settle and become trapped in their tangled roots and branches. This same process can even help them to accrete new soil, enabling their own upward and seaward migrations in a constant give-and-take with the sea. These ecosystems are also important for soaking up nitrogen, fueling their growth and, in so doing, reducing by 50 percent or more the amount of nitrogen that passes by into open waters. To date, there has been no global synthesis of these services from coastal wetlands.
In coastal bays, habitat-forming animal populations are also major contributors. Sponges of all shapes and sizes adorn the bottom of many coastal bays. In Florida Bay, sponges likely filtered the entirety of the bay every three days historically, maintaining clear waters that supported vast fish-producing seagrass meadows. A recent and dramatic reduction in sponge populations in Florida Bay has resulted in a four- to twenty-fold decrease in water clarity, and a marked decline in seagrass abundance, prompting both restoration of sponges and renewed calls for restoration “uphill” to reduce nutrient pollution entering the fresh water coming from the Florida Everglades.