Oyster reefs' control of carbonate chemistry-Implications for oyster reef restoration in estuaries subject to coastal ocean acidification.

Globally, oyster reef restoration is one of the most widely applied coastal restoration interventions. While reefs are focal points of processes tightly linked to the carbonate system such as shell formation and respiration, how these processes alter reef carbonate chemistry relative to the surrounding seawater is unclear. Moreover, coastal systems are increasingly impacted by coastal acidification, which may affect reef carbonate chemistry. Here, we characterized the growth of multiple constructed reefs as well as summer variations in pH and carbonate chemistry of reef-influenced seawater (in the middle of reefs) and ambient seawater (at locations ~50 m outside of reefs) to determine how reef chemistry was altered by the reef community and, in turn, impacts resident oysters. High frequency monitoring across three subtidal constructed reefs revealed reductions of daily mean and minimum pH (by 0.05-0.07 and 0.07-0.12 units, respectively) in seawater overlying reefs relative to ambient seawater (p < .0001). The proportion of pH measurements below 7.5, a threshold shown to negatively impact post-larval oysters, were 1.8×-5.2× higher in reef seawater relative to ambient seawater. Most reef seawater samples (83%) were reduced in total alkalinity relative to ambient seawater samples, suggesting community calcification was a key driver of modified carbonate chemistry. The net metabolic influence of the reef community resulted in reductions of CaCO3 saturation state in 78% of discrete samples, and juvenile oysters placed on reefs exhibited slower shell growth (p < .05) compared to oysters placed outside of reefs. While differences in survival were not detected, reef oysters may benefit from enhanced survival or recruitment at the cost of slowed growth rates. Nevertheless, subtidal restored reef communities modified seawater carbonate chemistry in ways that likely increased oyster vulnerability to acidification, suggesting that carbonate chemistry dynamics warrant consideration when determining site suitability for oyster restoration, particularly under continued climate change.

Warming and hypoxia reduce the performance and survival of northern bay scallops (Argopecten irradians irradians) amid a fishery collapse.

Warming temperatures and diminishing dissolved oxygen (DO) concentrations are among the most pervasive drivers of global coastal change. While regions of the Northwest Atlantic Ocean are experiencing greater than average warming, the combined effects of thermal and hypoxic stress on marine life in this region are poorly understood. Populations of the northern bay scallop, Argopecten irradians irradians across the northeast United States have experienced severe declines in recent decades. This study used a combination of high-resolution (~1 km) satellite-based temperature records, long-term temperature and DO records, field and laboratory experiments, and high-frequency measures of scallop cardiac activity in an ecosystem setting to quantify decadal summer warming and assess the vulnerability of northern bay scallops to thermal and hypoxic stress across their geographic distribution. From 2003 to 2020, significant summer warming (up to ~0.2°C year-1 ) occurred across most of the bay scallop range. At a New York field site in 2020, all individuals perished during an 8-day estuarine heatwave that coincided with severe diel-cycling hypoxia. Yet at a Massachusetts site with comparable DO levels but lower daily mean temperatures, mortality was not observed. A 96-h laboratory experiment recreating observed daily temperatures of 25 or 29°C, and normoxia or hypoxia (22.2% air saturation), revealed a 120-fold increased likelihood of mortality in the 29°C-hypoxic treatment compared with control conditions, with scallop clearance rates also reduced by 97%. Cardiac activity measurements during a field deployment indicated that low DO and elevated daily temperatures modulate oxygen consumption rates and likely impact aerobic scope. Collectively, these findings suggest that concomitant thermal and hypoxic stress can have detrimental effects on scallop physiology and survival and potentially disrupt entire fisheries. Recovery of hypoxic systems may benefit vulnerable fisheries under continued warming.