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Rangia as Potential Indicators of Bay Health
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Galveston Bay is an economically and ecologically important estuarine system on the Texas Coast sourced by freshwater inflows from an increasingly urbanized watershed. To regulate these flows, a baseline of ecological demands is established by monitoring biological response of estuarine organisms to changes in flows. Rangia clams have been identified as potential bioindicators for bay health. Historic rangia abundance and distribution data collected by TPWD and TCEQ showed that rangia were found in the greatest numbers in Trinity Bay and a decline in the overall population of Galveston Bay rangia throughout the past three decades. Though t-tests conducted on historical data showed that gear-related size exclusion significantly biased rangia CPUE and shell length data, smaller CPUE numbers in recent years compared to the rest of the historical record were supportive a genuine decline in rangia. After three years of present-day study (2012-2014), there was an observed increase in mean rangia shell length and decreases in mean meat index and areal density with a mean clam density of 25.3 (± 16.1) m^-2 in the Trinity River Delta and 22.5 (± 16.8) m^-2 in the Bay. Low mean monthly river discharges from the Trinity River during the study period complicated by drought and land use changes likely altered conditions which rangia require to for the survival of larvae and the initiation of spawning. These results also support the hypothesis that the low rangia densities found during the present-day study in Galveston Bay may be tied to the effects of drought conditions. PERMANOVA Main tests validated the comparability of the small-scale experimental design to long-term monitoring of bay wide sites by identifying significant variation in rangia abundance and health at different spatiotemporal levels. Multivariate analyses of clam health metrics and environmental parameters support a link between rangia health and variables influenced by freshwater inflow (salinity, DO, river discharge, dissolved nutrients) and explained one third of the variance in clam health metrics. Variables independent of FWI influence (temperature, water depth) were also related to clam health which further suggests that stressors unrelated to flows are compounding the effects of limited FWI on rangia.
Windham, Rachel (2015). Rangia as Potential Indicators of Bay Health. Master's thesis, Texas A & M University. Available electronically from