Freshwater Inflow Biotic Index (FIBI) for the Lavaca-Colorado Estuary, Texas

Freshwater inflow is an important source of physical variability in estuaries. Effects of water flow are dynamic, and it is impossible to sample all conditions as they vary over space and time. Benthos, however, are fixed in place, continuously sample the overlying water conditions, and demonstrate a variety of consistent responses to multiple sources of stress. Benthic indices of biotic integrity (BIBIs) have been particularly useful for assessing aquatic systems. However most indices have focused on assessing effects related to changes in water quality rather than water quantity. This study develops a Freshwater Inflow Biotic Index (FIBI) to determine how changes in freshwater inflow affect benthic populations, which in turn reflect the ecological condition of an estuary. Based on benthic succession theory and long-term data, 12 biotic metrics were chosen that characterized benthic community structure in response to inflow regimes. The metrics were ranked and then reduced to one variable using principal component analysis (PCA) to form the index. The FIBI and hydrological PC variables were significantly correlated, indicating that benthic communities respond to changes in salinity and do so in a relatively predictable manner. If inflow is reduced (i.e., salinity increased), it will cause upstream communities to take on characteristics of downstream communities. The FIBI successfully characterized effects of a salinity gradient in the Lavaca-Colorado estuary, and application of the FIBI approach should be successful in other estuarine ecosystems.