Vertical Distribution of Macrobenthos in Hypoxia-Affected Sediments of the Northern Gulf of Mexico: Applying Functional Metrics

Abstract

Continuing trends of declining bottom-water dissolved oxygen (BWDO) in coastal oceans are a threat to marine organisms, especially the benthos, and in the northern Gulf of Mexico (nGOM), the hypoxic area is expected to expand. Within-sediment vertical functional metric responses of macrobenthos were examined in four depth strata at four sites with different histories of exposure to seasonal hypoxia. The sites H7, D5, E4, and A6 between 30 and 39-m water depth represented a historical oxygen stress gradient from 5.4 to 2.1 mg L−1 BWDO (mid-summer 11-year mean), from which macrobenthos were sampled in spring and late summer 2009 and in mid-summer in 2010. High abundance together with high biomass and production potential within upper strata under low BWDO stress supports the primacy of a numerically driven response in this system. Total abundance decreased along the BWDO stress gradient, and the lowest abundance coincided at the highest stress (MaxStress) site together with the highest proportion of opportunists, the smallest mean size, and the shortest turnover time. Accordingly, the relative abundance of opportunists within the upper two strata increased while surface deposit feeders decreased with greater hypoxic stress. Notwithstanding the proportion of opportunists, functional metrics generally decreased across the BWDO stress gradient within the upper strata, and metrics generally varied inversely between upper and lower strata. The findings underscore how vertical benthos distribution patterns reflect the BWDO exposure regime and how secondary production varies as an indicator of trophic transfer potential and organic matter processing within this system.