ASSESSING THE EFFECTS OF ANTHROPOGENIC EUTROPHICATION USING LIVE-DEAD AGREEMENT IN MOLLUSCAN LIFE HISTORY TRAITS

Given a trade-off between the size and number of eggs that a mother can produce, life history theory predicts that increased food availability will lead to increased juvenile survival and consequently a reduction in mean egg size over time. Previous studies have shown that the egg sizes of suspension-feeding marine mollusks can evolve in response to regional shifts in nutrient concentrations over geologic time. Have anthropogenic changes in the delivery of nutrients to coastal ecosystems led to similar changes in molluscan egg size over much shorter time scales? We address this question by comparing the sizes of bivalve larval shells (PI) in present-day populations in the northern Gulf of Mexico with those preserved in associated death assemblages. PI size is positively correlated with egg size and can be measured from adult shells. We focus on the direct-developing bivalve Nuculana acuta because it is relatively common live and dead in subtidal samples collected along a gradient in anthropogenic eutrophication that extends eastward from the Louisiana Bight to the Mississippi-Alabama barrier islands. Live-dead comparisons for samples collected offshore Dauphin Island, AL show no significant difference between present-day mean PI size and the mean PI size of the death assemblage (Mann-Whitney U-test, p > 0.05). Furthermore, radiocarbon ages for measured specimens from this site show no trend in PI size over the past 3000 years (Spearman rank order correlation, p > 0.05). PI size tends to be underestimated as the quality of shell preservation declines, which in the death assemblage can be due to a diversity of taphonomic processes and among live-collected individuals can occur when shells are preserved in corrosive ethanol. The results presented here are robust to such variation in shell condition. The live-dead agreement that we observe in the life history of N. acuta may reflect the relatively recent onset of eutrophic conditions reported for localities on the continental shelf seaward of the Mississippi-Alabama barrier islands. If so, these data provide a baseline for evaluating live-dead agreement in populations in the Louisiana Bight that have historically experienced more extensive, and persistent, eutrophic conditions.