Estuaries are productive, complex and have great economic value by virtue of their fisheries, ecosystem services and recreation potential. They are typically less buffered to acid than open oceans due to the combined effects of acid production during heterotrophy and acidic inputs from both land and atmosphere. Within estuaries, it is important to understand how varying acid burdens impact living resources, particularly those that provide ecosystem services and/or generate income as fisheries. The bivalve Mya arenaria, the focal species of this proposed research, is one such resource that sustains a valuable coastal fishery while providing service via its filtration capacity. Because Mya shells are constructed from a relatively soluble form of calcium carbonate (aragonite), and the clams often inhabit eutrophic waters, they may be particularly vulnerable as pH declines. Planktonic larvae and benthic juveniles are critical life stages -- even small reductions in theier abundances could substantially decrease adult populations.
This proposed research addresses four distinct hypotheses concerning the roles of riverine and sediment interactions on the viability of larval and juvenile Mya. Research activities include the following.
1. Fieldwork will evaluate the spatial and seasonal changes in aragonite saturation state within the Kennebec River Estuary and Casco Bay. Seasonal sampling will be coupled with high-frequency sampling during the annual Mya spawn to observe and document the effect of lowered aragonite saturation state on the health status of larval Mya.
2. Using larval Mya, laboratory experiments will mimic the aragonite saturation state observed in Casco Bay during the high-frequency cruises. Metamorphic change (veligers, pediveligers, and metamorphosed juveniles), growth rate, and survivorship of Mya will be evaluated as a function of aragonite saturation state.
3. Spatially intensive daily cohort monitoring of the intertidal mud flats in Falmouth, Maine, will establish the link between changes in abundance of settling juveniles and aragonite saturation state during the period of Mya set. Cohort monitoring of settling Mya will be examined in reference to sediment pH and aragonite saturation state in nearby deposits to ascertain if sediment saturation state is a primary settlement cue for transitioning larvae.
4. A diagnostic model will be developed for the shellfish management community that can be used to detect aragonite saturation state of the water column. The model would run on routine oceanographic measurements (salinity, temperature, oxygen and chlorophyll fluorescence).
Intellectual merit of study: The chemical consequences of increasing atmospheric CO2 and resulting hydrolysis of carbonic acid is well understood and resultant ocean acidification has been accurately predicted with the current generation of global circulation models. These predictions have accelerated research into the effects of ocean acidification on marine organisms, particularly those with CaCO3 exoskeletons. Estuarine waters are far less buffered than oceans, are subject to a variety of acid loadings, and are quite possibly acidifying at a faster rate than the open ocean. Yet, these regions have been largely ignored in 'acidification' research. Effects of acidification on calcifying organisms are similar regardless of whether of acid origin -- atmospheric exchange, net heterotrophy, or discharge of acidic river water. Likewise, each of these acid fluxes is being perturbed via anthropogenic activity (e.g. fossil fuel use, deforestation, agriculture). The proposed research will further understanding of the combined and cumulative impacts of varied acid burdens on calcifying organisms in coastal waters.
Broader impacts of study: Saint Joseph's College of Maine is an undergraduate institution with many potential opportunities to introduce students to the excitement of scientific research and discovery. This project will have special significance to the 70% of women that make up the student body at Saint Joseph's College, many of whom will experience research science for the first time. The immersion of students into research will be modeled after other sponsored research at SJC, resulting in a new marine science major, in over 35 SJC undergraduates attending and/or presenting research results at national/international conferences (12 as co-authors), and in 4 students co-authoring recent papers for publication.
This project looked at the role of river input in altering pH and controlling carbonate mineral saturation states in Casco Bay Estuary, Gulf of Maine. In addition, we evaluated the role of sediment geochemistry on the recruitment success of small, commercially important bivalves (the soft shell clam, Mya arenaria) in a mud flat in the estuary. Through both field and laboratory work we showed that river input significantly depresses the buffering capacity of the recieving water, lowering carbonate saturation states to levels which impart significant stress on planktonic clam larvae. We also showed that surface sediments in coastal mud flats of Maine are sites of intense carbonate minerral dissolution due to the production of metabolic acids. Small clams can actually dissolve in some regions. Further offshore we document the tight coupling of CO2 consumption and production in surface and bottom water, respectively, and the associated changes in saturation state in these distinct zones during summer-time water column stratification. During summer, photosynthesis in the photic zone consumes CO2, raising both pH and saturation states. This occurs during the time when larval clams are in the surface water and may provide for thermodynamically favorable conditions for their growth and survivorship. Conversely, the bottom water, decoupled from the surface water due to the formation of a strong thermocline, shows a corresponding significant increase in CO2 and decreases in pH and saturations states, making these waters far less thermodynamically favorable to carbonate bearing fauna. Turnover during spring and fall homogenizes the chemistry of the water column.
