Life Histories Vary with Primary Productivity in the Northern Gulf of Mexico

Human activities have increased nutrient delivery to aquatic ecosystems around the world, spurring primary productivity, and leading to the establishment and expansion of oxygen-limited “dead zones.” How will marine animals respond to these changing conditions? To address that question, we take a space-for-time approach and compare the traits of different marine invertebrates along a primary productivity gradient in the northern Gulf of Mexico. Previous studies have found that life history traits can be sensitive to prevailing environmental conditions. Using Holocene death assemblages collected from -20 meters offshore Alabama and Florida, we test the hypothesis that bivalve egg size, and bryozoan reproductive mode, vary with primary productivity. Based on previous studies, we expect populations in areas with an abundance of food to exhibit the following characteristics: 1) cupuladriid bryozoans will exhibit greater frequencies of clonal to aclonal reproduction; and 2) bivalves will produce smaller eggs due to greater juvenile survivorship and fecundity selection. We found that Discoporella depressa colonies show low frequencies of clonal reproduction overall, but that percent clonality was greater in coastal Alabama than Florida. Cupuladria colonies showed higher proportions of clonal reproduction, whereas Reussirella doma colonies exhibited exclusively aclonal reproduction. Egg size is positively correlated with the earliest stage of larval shell growth (PI size) in marine bivalves. Nucula proxima larval shell size varied inversely with primary productivity; larval shells were larger in Florida than Alabama. Preliminary live-dead results in both regions show limited evidence of change over time, in contrast with previous analyses of other bivalve species in the region. These space-for-time case studies highlight ways in which benthic marine invertebrates may respond to future anthropogenic driven changes in primary production in the coastal ocean.