Water quality and carbonate chemistry dynamics in an estuarine system impacted by mariculture

Cultural eutrophication due to the increasing demand of an expanding population has posed negative impacts on estuarine systems worldwide. In tropical regions, the variability in the inputs of nutrients and organic matter from rivers to coasts is further influenced by the changing monsoon seasons. This study examined the water quality and carbonate chemistry in a semi-enclosed estuary in the northwest Philippines that is used for farming milkfish and other aquaculture species. Data suggests that the mariculture area is a heterotrophic system enriched in dissolved inorganic carbon and pCO₂ but depleted of nitrate due to the decomposition of unconsumed and undigested fish feeds from mariculture activities. The different river systems surrounding the estuary act as nitrate sources that could relieve nitrogen limitation during the wet season. Results also show hypoxic conditions not only in mariculture waters but in river systems as well. Accounts of the overflow of hypoxic river waters to the mariculture area could potentially provide a different mechanism of fish kill occurrence and requires further scientific observations. pH data confirm fast rates of coastal acidification in mariculture waters due to organic matter decomposition to levels that are expected to be experienced by the end of this century in open ocean conditions considering the air-sea equilibrium of increasing atmospheric CO₂. These results highlight the need for more advanced biogeochemical and transdisciplinary investigations of these transition zones and their implications on climate, biodiversity, and sustainability.