A Multivariate Statistical Analysis of Relationships Between Freshwater Inflows and Mollusk Distributions in Tidal Rivers in Southwest Florida

Abstract

The estuaries and rivers of the western coast of Florida, bordering the Gulf of Mexico, has been under intense study for some time with a goal to identify relationships between inflows, salinity, and natural resources. The mollusks have been show to be especially sensitive to salinity in many past studies, in many parts of the world. Several recent studied supported by the Southwest Florida Water Management District have focused on mollusk distributions for six tidal rivers: Peace River, Alafia River, Myakka River, Weeki Wachee River, Shell Creek, and the Shakett Creek Dona/Roberts Bay system. The purpose of the current project is to perform an inter-river, multivariate analysis that examines relationships between freshwater inflows, physicochemical variables that are affected by freshwater inflows (e.g. salinity, dissolved oxygen), and the distribution of mollusk populations in tidal rivers of southwest Florida.

The design of all studies consists of mollusks being sampled along transects within each river system. The transects run lengthwise originating at the mouth of each river, heading upstream. To enable all of the rivers to be compared simultaneously, the measure of distance along each transect was standardized by grouping all stations along each transect into two-kilometer (2-km) segments. Community structure of mollusk species was analyzed using non-metric multi-dimensional scaling (MDS). Relationships between mollusk communities and environmental factors were identified by using a mulitvariate procedure that matches biotic (i.e., mollusc community structure) with environmental (i.e., sediments, temperature, dissolved oxygen, salinity and, pH) variables. Analyses were constrained to variables that were common to all data sets.

In this limited analysis of southwest Florida mollusk communities, it is concluded that mollusk species are controlled more by water quality rather than the sediment they live in or on. The most important variable correlated with mollusk communities is salinity, which is a proxy for freshwater inflow. It is almost impossible to directly link community changes in response to inflow changes, because not replicates over time were carried out in the rivers sampled. Although total mollusk abundance was not a good indicator of inflow effects, certain indicator species have been identified however, that characterize salinity ranges in southwest Florida rivers. Corbicula fluminea, Rangia cuneata, and Neritina usnea were the only common species that occurred at salinities below 1 psu. Although, C. fluminea was the best indicator of freshwater habitat, because densities were highest below 2 psu, it is an introduced bivalve species. Rangia cuneata, a bivalve, has been noted as an indicator of a fresh- to brackish-water with an estimated tolerance of up to 20 psu in other studies as well. Neritina usnea is a gastropod and is also common in fresh- to brackish-water salinities. These salinity ranges may be useful in predicting mollusk community reactions to alterations in salinity that result from actual or simulated changes in freshwater inflow.