Center for Coastal Studies CCBNEP Living Resources Report Texas A&M University-Corpus Christi Results - Open Bay Habitat B. Living Resources - Habitats Detailed community profiles of estuarine habitats within the CCBNEP study area are not available. Therefore, in the following sections, the organisms, community structure, and ecosystem processes and functions of the major estuarine habitats (Open Bay, Oyster Reef, Hard Substrate, Seagrass Meadow, Coastal Marsh, Tidal Flat, Barrier Island, and Gulf Beach) within the CCBNEP study area are presented. The following major subjects will be addressed for each habitat: (1) Physical setting and processes; (2) Producers and Decomposers; (3) Consumers; (4) Community structure and zonation; and (5) Ecosystem processes. HABITAT 1: OPEN BAY Table Of Contents Page 1.1. Physical Setting & Processes ............................................................................ 45 1.1.1 Distribution within Project Area ......................................................... 45 1.1.2 Historical Development ....................................................................... 45 1.1.3 Physiography ...................................................................................... 45 1.1.4 Geology and Soils ................................................................................ 46 1.1.5 Hydrology and Chemistry ................................................................... 47 1.1.5.1 Tides .................................................................................... 47 1.1.5.2 Freshwater Inflow ............................................................... 47 1.1.5.3 Salinity ................................................................................ 48 1.1.5.4 Nutrient Loading .................................................................. 49 1.1.5.5 Nutrient Distributions........................................................... 49 1.2 Producers & Decomposers ................................................................................ 50 1.2.1 Primary Producers .............................................................................. 50 1.2.2 Decomposers ...................................................................................... 51 1.3 Consumers ......................................................................................................... 52 1.3.1 Invertebrates ....................................................................................... 52 1.3.1.1 Zooplankton ......................................................................... 52 1.3.1.2 Meiobenthos ........................................................................ 53 1.3.1.3 Macrobenthos....................................................................... 53 1.3.1.4 Epibenthos ........................................................................... 55 1.3.2 Fish ..................................................................................................... 55 1.3.3 Reptiles & Amphibians ....................................................................... 56 1.3.4 Birds ................................................................................................... 56 1.3.5 Mammals ............................................................................................ 57 1.4 Community Structure & Zonation .................................................................... 57 1.4.1 Planktonic Communities .................................................................... 57 1.4.1.1 Phytoplankton ...................................................................... 57 1.4.1.2 Zooplankton ......................................................................... 59 1.4.1.3 Summary .............................................................................. 61 43 Center for Coastal Studies CCBNEP Living Resources Report Texas A&M University-Corpus Christi Results - Open Bay Habitat Page 1.4.2 Benthic Communities ......................................................................... 62 1.4.2.1 Salinity Effects on Horizontal Distribution of Benthos ...... 62 1.4.2.2 The Effects of Variation in Freshwater Inflow on Horizontal Benthic Distributions .......................................... 67 1.4.2.3 Seasonal Variation in Macrobenthos .................................. 69 1.4.2.4 The Effects of Sediment Type on Horizontal Distributions of Benthos ....................................................... 70 1.4.2.5 The Effects of Biological Interactions on Benthic Horizontal Distributions ....................................................... 72 1.4.2.6 Vertical Distributions of Benthic Fauna ............................. 73 1.4.3 Nektonic Communities ...................................................................... 74 1.4.3.1 Data Sets ............................................................................. 74 1.4.3.2 Habitat Partitioning ............................................................. 75 1.4.3.3 Freshwater Inflow and Fisheries Harvests ......................... 79 1.5 Ecosystem Processes ......................................................................................... 80 1.5.1 Energy Flow ....................................................................................... 80 1.5.1.1 Primary Production ............................................................. 81 1.5.1.2 Secondary Production ......................................................... 83 1.5.2 Trophic Levels and Food Web Relationships .................................... 84 1.5.3 Nutrient Cycling ................................................................................. 86 1.5.3.1 The Process of Regeneration ............................................... 87 1.5.3.2 Nitrogen Losses .................................................................. 88 1.5.3.3 Factors Affecting Regeneration .......................................... 90 1.5.3.4 Nutrient Process Zones ...................................................... 92 1.5.4 Linkages with Other Systems ............................................................ 94 Literature Cited ........................................................................................................ 96 44 Center for Coastal Studies CCBNEP Living Resources Report Texas A&M University-Corpus Christi Results - Open Bay Habitat HABITAT 1: OPEN BAY 1.1 Physical Setting and Processes 1.1.1 Distribution within Project Area The open bay community is defined as the unvegetated and soft-bottomed portion of the subtidal estuarine environment. This community is the dominant subtidal community in Texas estuaries comprising 68% of the total subtidal area (Armstrong, 1987). Extent of the open bay community is determined primarily by factors limiting success of submerged plants and oysters such as depth, turbidity, exposure to wave action, and salinity. Most of Corpus Christi Bay, Nueces Bay, Oso Bay, Mission Bay, and Aransas Bay, except for a few scattered areas of oyster reefs and seagrass meadows, can be characterized as open bay. Extensive reefs of Crassostrea virginica (eastern oyster) limit the open bay community to inter- reef areas in Copano and Nueces Bays. Extensive grass beds limit the open bay community in the shallower Redfish Bay and Laguna Madre. Seagrass meadows have greatly expanded in Oso Bay since the Barney Davis Power Plant began introducing Laguna Madre water into this system. 1.1.2 Historical Development In geologic terms the open bay community is an intermediate stage along a successional continuum that will end in the ultimate filling of the estuary and creation of flat coastal plains. Rivers continue to deposit sediment, as estuaries become shallower and smaller in areal extent. Sediment accumulation is slow, perhaps a few centimeters per 100 years, but the ultimate fate of these estuaries is illustrated by coastal plain farmlands that were once the Brazos and Rio Grande estuaries. The Mission/Aransas and Nueces estuaries and Baffin Bay formed from the combination of two processes: drowning of river valleys and formation of barrier islands. Present geomorphology of estuarine systems of the study area depict the geologic past. Morphology of bays that are oriented perpendicular to the coastline (e.g., Corpus Christi, Nueces, and Baffin bays) may be the result of drowning of ancestral river valleys, whereas general morphology of those bays oriented parallel to the coastline (e.g., Aransas Bay and Laguna Madre) may be explained with reference to formation of barrier islands. 1.1.3 Physiography Estuarine systems of the CCBNEP study area, classified as predominately open bay, include the Nueces and Mission/Aransas estuaries and encompass 14.5% of estuarine areas of Texas. The Nueces Estuary, comprised of Nueces, Oso and Corpus Christi bays with a total surface area of 44,451 ha, represents 7.13% of total estuarine area in Texas. Average depth for the entire system is 2.4 m., yielding a volume of 1.147 km3 (Armstrong, 1987). Corpus Christi Bay (deepest bay in the study area) is 4.2-4.8 m deep over most of the bay area; bay margins are generally steeply sloped (White et al., 1983). Oso and Nueces bays are shallower, ranging from 45 Center for Coastal Studies CCBNEP Living Resources Report Texas A&M University-Corpus Christi Results - Open Bay Habitat 0.6 to 1.2 m respectively. Nueces Estuary has access to Gulf waters through Aransas Pass which traverses the barrier island to the northeast, and to freshwater from Nueces River which flows into Nueces Bay. The Mission/Aransas Estuary is