Habitat Suitability Modeling and Mapping to Assess the Influence of Freshwater Withdrawals on Spatial Distributions and Population Numbers of Estuarine Species in the Lower Peace River and Charlotte Harbor, Florida

Abstract

The effects of potential reductions of freshwater inflow were evaluated for the lower Peace River and its largest tributary, lower Shell Creek, which flow into the Charlotte Harbor estuary in southwest Florida. Habitat suitability modeling (HSM) and mapping of fish and invertebrate species life stages were used to seasonally predict changes in spatial distributions and population numbers associated with simulated freshwater withdrawals. Seasonal salinity grids and temperature grids derived from values predicted by hydrodynamic modeling (2007-2014) were similar between baseline (i.e., flows not affected by water withdrawals) and minimum flows (flows associated with water withdrawals). Depth grids, bottom type grids, and seasonal dissolved oxygen grids were held constant between the two scenarios. Seasonal habitat suitability models were applied to 28 fish and invertebrate species life stages with affinities for low or moderate salinity. Salinity was the most significant factor in seasonal models for species life stages. The seasonal HSM maps produced showed that spatial distributions were similar between baseline and minimum flows for each species life stage. Most seasonal estimates of population numbers under minimum flows were less than the estimates for the baseline condition, indicating some effect on population numbers associated with flow reductions. Reductions in population numbers under minimum flows ranged from 0.3% to 21.0%, with 3 out of 28 seasonal comparisons indicating losses >15% and 12 comparisons indicating losses between 5% and 15%. Although other factors related to freshwater inflow can also influence species abundance and distribution, these results demonstrate how output from hydrodynamic modeling can be applied to HSM analyses and mapping to estimate spatial changes in habitat areas and population numbers for the life stages of selected fish and invertebrate species in relation to changes in salinity distributions, which can be used to identify areas of an estuary that are particularly susceptible to the effects of inflow reductions. The assessment and management of freshwater inflow to estuaries have received increased emphasis in recent decades to account for the important ways in which freshwater inflow affects physical, chemical, and biological processes and the resources of estuaries, including relationships with the productivity of sport and commercial fish

Development of modelling and mapping methods to predict spatial distributions and abundance of estuarine and coastal fish species life‐stages in Florida

“…Salinity was significant during most seasons for the species life‐stages listed for both studies in CH (Rubec et al., 2019, 2021). The optimum zones in seasonal HSM maps for the first six species life‐stages, classified as estuarine residents, indicate that they were most abundant in either low or moderate salinities in the lower Peace River during most of the year (Rubec et al., 2019).…”

“…A contract was obtained from the SWFWMD in 2015 to assess the impact of potential freshwater withdrawals from the tidally influenced lower Peace River and CH. Delta gamma GAMs were applied with seasonal modelling of eight species life‐stages in the CH study area with highest abundances in low to moderate salinities (Rubec et al., 2019, 2021). The species life‐stages analyzed in 2015–2016 are presented in Table 2.…”

“… Note : Percent changes in population numbers between flow conditions: decreasing (regular font) and increasing (bold font) (Rubec et al., 2021). …”

“…Fitted splines were created using delta‐gamma GAMs for both positive CPUE (+CPUE) data (Mu) and probability of zero occurrence data (Nu) across environmental gradients for each species life‐stage analyzed by season in CH (Rubec et al., 2019, 2021). The CPUEs for each gear type were gear‐corrected and combined (Mu X Nu) within the HSM to create GC‐CPUE splines for each environmental factor and histograms for bottom type, season, and year.…”

“…The depth, bottom type, and dissolved oxygen grids used were the same in both studies. Hence, the lower population estimates in the 2021 study appear to be related to the seasonal temperature grids and seasonal salinity grids derived from data produced by hydrodynamic modelling and used for the Baseline and Minimum Flows analyses (Rubec et al., 2021). The most likely explanation is that the higher population number estimates in the 2019 study were related to generally higher rainfall and freshwater inflows that occurred prior to 2007, which influenced seasonal salinity grids (Rubec et al., 2019).…”