Aim: A 21-year fisheries-independent monitoring dataset was used to explore fish community diversity across a latitudinal gradient to quantify how diversity has changed and relate those changes in diversity to changes in the abiotic environment. Additionally, this study spans a biogeographic transition zone, providing insight into future species assemblages across regions of relatively high species diversity.

Location: Indian River Lagoon, Florida, USA

Methods: Spatial and temporal beta diversity was quantified latitudinally with “best derived breaks” determined by using chronological cluster analyses.  Multiple indices of alpha diversity were quantified, including species richness, Shannon diversity, Simpson diversity, and Pielou’s evenness. AIC model selection and environmental fit tests were performed to link patterns of diversity and species assemblages with the abiotic environment.

Results: Evidence of a biogeographic transition zone was supported by data spanning the entire study period; the largest break in species assemblage occurred near 28°N. Fine scale analyses using small and large seine catches were noisier than broad analyses but indicated a northern shift in location of the biogeographic transition zone. Beta-diversity was generally dominated by species turnover/balance versus nestedness/gradient components, implying that changes were driven by species sorting associated with the physical environment. Excluding the summation of all environmental variables, temperature and dissolved oxygen best describe patterns of diversity and species composition.

Main Conclusions: Over years less affected by disturbances, large and small seine catch data suggest the fish community assemblage and location of the biogeographic transition zone has shifted 9 km and 21 km to the north. If the trends observed in these years were to continue from 1999 until the year 2100, a 111 km to 243 km shift in fish communities could be expected. Variation in rates of movement based on gear type suggest novel species assemblages could ensue.

Samples were collected, and data generated by the Florida Fish and Wildlife Conservation Commission (FWC) Melbourne - Indian River Field Laboratory as part of the state-wide Fisheries Independent Monitoring (FIM) program.  A description of the random stratified sampling FIM program can be found at: https://myfwc.com/research/saltwater/reef-fish/monitoring/fim-stratified-random-sampling/).  FIM data can be requested through the contact information for the Melbourne lab on the following website: https://myfwc.com/research/about/connect/locations/).  Fish were sampled monthly with bag seines and a large haul seine using a stratified random sampling design. Bag seines were 21.3m long, dragged for 15.5m and used to collect juvenile and small adult fish (typically <10cm) in areas having less than 1.5 m of water. Haul seines were 183 x 3m, deployed by boat in a rectangular shape along shorelines and on offshore flats, and used to collect larger adult fish (Stevens et al., 2016). Fish were identified and enumerated in the field and released, with a subset of samples returned to the lab to verify accuracy of identification. At the time of collection, related environmental variables were recorded including temperature, dissolved oxygen, salinity, pH, and conductivity using Hydrolab and YSI units.