Fish size structures in lakes of the Lower Mississippi River floodplain

Abstract The Lower Mississippi River has a floodplain that includes >1350 perennial lakes carved by shifts in river courses and other hydro‐fluvial processes over eons. Notwithstanding their similar provenances, these waterbodies exhibit an immense variety of morphologies and successional stages that illustrate their natural trajectory from aquatic to forested wetlands. A result of this geographical, morphological and temporal diversity is dynamic and varied fish communities. We examined how size structures of fish communities in these floodplain lakes were associated with key in‐lake and off‐lake environmental drivers. Fish lengths were collected with standardised procedures in a sample of 30 of these lakes to construct a lake‐by‐length group matrix. Likewise, in‐lake and off‐lake environmental descriptors were collected to construct a lake‐by‐covariate matrix. Distance‐based linear models were used to assess associations between fish size structure and environmental descriptors. Smaller fish were typically associated with increasing levels of turbidity, chlorophyll‐ a , phycocyanin and surrounding agriculture. Shallow, hypereutrophic floodplain lakes associated with agricultural landscapes and reduced connectivity experience harsh physicochemical environments. These conditions appeared to hinder the formation of sustained fish communities but may confer a survival advantage to juveniles or small short‐lived species. Conversely, larger fish were associated with increasing lake depth, water clarity, connectivity, and extent of surrounding forests‐wetlands. Enhanced stability and size structure were observed in communities residing in deeper and clearer lakes, suggesting that these conditions facilitated the development of longer‐lived species spanning multiple age groups. The enhanced connectivity that facilitated this increased stability also permitted the presence of larger itinerant species. Size‐structure assessments can serve as a valuable ecological and biodiversity indicator in floodplain lakes. Size‐structure assessments could supplement and, depending on objectives, even supplant conventional taxonomic analyses, and enhance surveillance of this vast and important natural resource.