chapter  20
28 Pages

Effect of Freshwater Inflow on Nutrient Loading and Macrobenthos Secondary Production in Texas Lagoons

The Texas coast is characterized by lagoons behind barrier islands, and their ecology is strongly influenced by a longitudinal ecotone where freshwater inflow and nutrients decrease from northeast to southwest. The four Texas lagoons studied here are the Lavaca-Colorado, Guadalupe, Nueces, and Laguna Madre estuaries. Each of these lagoonal estuaries consists of a primary bay (the lagoon), which is nearest to the Gulf of Mexico, and a secondary bay, which is nearest to the freshwater influent source. The hypothesis of the current study is that climatic variability and inflow differences

Abstract .......................................................................................................................................... 513 20.1 Introduction .......................................................................................................................... 514 20.2 Conceptual Model ................................................................................................................. 514 20.3 South Texas Lagoonal Estuaries ........................................................................................... 515

20.3.1 Lavaca-Colorado Estuary ......................................................................................... 517 20.3.2 Guadalupe Estuary ................................................................................................... 517 20.3.3 Nueces Estuary ......................................................................................................... 518 20.3.4 Laguna Madre Estuary ............................................................................................. 518

20.4 Materials and Methods ......................................................................................................... 519 20.4.1 Study Sites ................................................................................................................ 519 20.4.2 Databases .................................................................................................................. 520

20.4.2.1 Benthos ...................................................................................................... 520 20.4.2.2 Hydrography .............................................................................................. 520 20.4.2.3 Predators .................................................................................................... 520 20.4.2.4 Primary Production.................................................................................... 520 20.4.2.5 Hydrology .................................................................................................. 520

20.5 Modeling ............................................................................................................................... 520 20.5.1 Calibration and Validation ........................................................................................ 526

20.6 Results ................................................................................................................................... 528 20.7 Discussion ............................................................................................................................. 532 Acknowledgments .......................................................................................................................... 537 References ...................................................................................................................................... 537

among the ecosystems alter nutrient loading, which in turn affects benthic infaunal communities and maintains secondary production. Macrobenthic biomass data from a 5-year period were used to calibrate a bioenergetic model of secondary production. Benthic organisms were divided into two trophic groups: deposit feeders (that consume detritus or sediment organic matter) and suspension feeders (that filter phytoplankton or graze on benthic diatoms). Community structure is controlled by inflow, with more suspension feeders in high inflow estuaries and more deposit feeders in low inflow estuaries. Higher inflow translates into higher productivity. Within estuaries, the production to biomass ratio (P/B, unit of 1/year) increased with proximity to the freshwater source. The P/B ratio increased with water residence time (i.e., inflow volume adjusted by the estuary volume). The one exception was Laguna Madre, with the highest P/B of 3.2, which is due to extensive seagrass habitat. Corpus Christi Bay, with little inflow, had the lowest P/B of 1.2. Results of this study show that freshwater inflow, and concomitant nutrient loads, drives benthic community structure and function.