Anthropogenic disturbances such as oil spills can cause mortality in benthic infaunal communities, reducing diversity and abundance and impeding sediment ecosystem functions. Sublethal effects of oil exposure have received less attention, however. We conducted a mesocosm experiment exposing 2 infaunal taxa, the polychaete Owenia fusiformis and the brittle star Hemipholis elongata, to sublethal concentrations of the water-accommodated fraction (WAF) of oil. We evaluated the effects of WAF on animal behavior, bioturbation, and sediment oxygen demand (SOD) in infaunal assemblages of both mixed and single species. WAF exposure did not affect O. fusiformis feeding behavior, nor did it influence bioturbation. Compared to O. fusiformis, the brittle star H. elongata mixed more surface sediments to greater depths and to a greater extent horizontally. Bioturbation in mesocosms with both taxa was consistent with predictions from monocultures for substitutive densities, but lower than predicted for additive densities. This indicates that taxa interacted (negatively) only at higher densities. SOD was higher in oiled than unoiled treatments initially (at 1-3 d), but this difference disappeared after the first sampling, consistent with a decrease in total petroleum hydrocarbons in the WAF treatment over the same time period. Higher SOD in WAF-exposed faunal treatments than sediments with no fauna suggested that faunal activities may enhance microbial degradation of hydrocarbons. These findings suggest that exposure to WAF stimulated microbial metabolism in the first few days of the experiment, but did not affect macrofaunal behavior/function, nor have lasting effects on sediment ecosystem functions.

Infaunal sedimentary communities underpin marine ecosystems worldwide. Understanding how disturbances such as oil spills influence infauna is therefore important, especially given that oil can be trapped in sediments for years or even decades. The 2010 Deepwater Horizon (DWH) event was the largest marine oil spill in United States history, impacting habitats throughout the Northern Gulf of Mexico. We investigated infaunal community structure at two shallow sites in the Chandeleur Islands, LA, United States, over a 2-year period from 2015 to 2016 (5–6 years post-spill). One site was moderately contaminated with oil from the DWH spill, while the other was only lightly contaminated. Both sites featured patchy Ruppia seagrass meadows, allowing us to compare infaunal communities between sites for seagrass versus unvegetated sediment. The moderately-oiled site featured a significantly different community than that of the lightly oiled site; these differences were driven by altered abundance of key taxa, with some taxa being less abundant at the moderately oiled site but others more abundant. During our second year of sampling, a crude oil slick moved transiently through the moderately-oiled site, allowing us to directly observe responses to an acute re-oiling event. Virtually every taxonomic and community-level metric declined during the re-oiling, with effects more pronounced in seagrass beds than in unvegetated sediment. The sole exception was the snail, Neritina usnea, which we found exclusively at the more-oiled site. Our observations suggest that oil responses are driven more by key taxa than by entire guilds responding together. By identifying the families and genera that showed the largest signal at this pair of sites, we can begin laying groundwork for understanding which benthic taxa are most likely to be impacted by oil spills, both in the immediate aftermath of a spill and through longer-term contamination. While more studies will certainly be needed, this contribution is a step towards developing clear a priori hypotheses that can inform future oil-spill work. Such hypotheses would help to focus future sampling efforts, allowing resources to be directed towards those taxa that are most likely to be responding, and which are potential bio-indicators of oil exposure.

Shallow marine soft sediments serve an important function by remineralizing organic matter, which consumes dissolved oxygen in the process. Sediment oxygen demand (SOD) depends on overlying water oxygen concentration but can also be altered by mixing and irrigating activities of sediment macroinfauna. Oxygen patterns in shallow coastal waters can vary substantially on short time scales, frequently following a diel cycle. However, little is known about whether and how the presence of macrofauna in sediment may affect SOD throughout a diel cycle. In this field study, we examined how sediment oxygen demand varied over a diel oxygen cycle and with sediment macroinfaunal presence. In situ flow-through benthic metabolism chambers were used to measure SOD at a high temporal resolution in discrete sediment patches. We found that sediments with more macroinfauna had greater average SOD over the diel cycle, consistent with previous studies. There was an interaction between the effects of faunal biomass and DO on SOD, suggesting that where macroinfauna are present they drive greater SOD during nightly low oxygen, presumably by enhancing their irrigation. SOD was also highly variable on a sub-diel timescale, and more variable in sediments with more macroinfauna. This indicates that sediment oxygen demand is dynamic and highly sensitive both temporally, on very short timescales, and spatially, in terms of resident fauna. High temporal and spatial resolution measurements, particularly on the diel scale, are critical to accurately estimate sediment metabolism.

Changes in dissolved oxygen concentration can cause dramatic shifts in chemical, biological, and ecological processes in aquatic systems. In shallow coastal areas, this can happen on short timescales, with oxygen increasing during the day due to photosynthesis and declining at night due to respiration. We present a system controlled by an Arduino microprocessor that leverages the oxygen-consuming capacity of sediments to manipulate dissolved oxygen in an aquarium tank to planned concentrations. With minor adjustments to the Arduino code, the system can produce a variety of dissolved oxygen patterns, including a diel cycle. Designed to be user-friendly and scalable if needed, the system uses easily acquired, low-cost electronic and aquarium components. Its simplicity and accessibility permit deeper exploration of the effects of dissolved oxygen variability in aquatic systems, and the use of Arduino code and basic electronics makes it a potential tool for teaching experimental design and instrument fabrication.