Turbidity influences trophic interactions in estuaries

We investigated how changes in abiotic conditions resulting from human activities indirectly alter trophic interactions using turbidity in estuaries as a model system. Development and nutrient input are causing turbidity to increase in many coastal areas. Using an 18 yr data set from Aransas and San Antonio Bays in Texas, we found fish abundance (Sciaenops ocellatus, Pogonias cromis, Archosargus probatocephalus) to be highest in low turbidity (< 30 nephelometric turbidity units [NTU]; p < 0.01), while crab (Callinectes sapidus) abundance was highest in high turbidity (> 30 NTU; p < 0.05). In field studies, mud crabs (Panopeus spp.), an important intermediate predator on oyster reefs that are not targeted in the 18 yr data set, were more abundant on oyster reefs in St. Charles Bay, Texas, when turbidity exceeded 30 NTU (p = 0.03). Fish predation on tethered Panopeus herbstii was greatest when turbidity was low (< 30 NTU, p < 0.05), but predation by crabs (p = 0.003) and overall predation (p = 0.02) were greatest in high turbidity (> 30 NTU). Predation on oyster spat was not different between low- and high-turbidity sites (p = 0.64). However, oysters devoted more resources to shell growth (p < 0.01) at a cost of less somatic growth and fecundity, a reaction known to occur in response to crab predators. Elevated turbidity can alter trophic interactions in estuaries by altering species composition and trophic interactions, leading to an increase in crab abundance, which can alter predation rates as well as growth in juvenile oysters.