Award: OCE-1255042

Oceanic uptake of anthropogenic CO2 is changing the acid-base balance of the oceans towards increasing acidity. This could negatively affect marine organisms and ecosystems, as well as human communities dependent on resources and services provided by the sea. It is therefore important to understand how seawater chemistry will change in response to continued anthropogenic CO2 emissions. In open ocean surface seawater, the acid-base balance can be reasonably well predicted based on fundamental chemical principles, but in near shore environments it is necessary to also consider the influence from local biogeochemical processes, including net ecosystem organic carbon production (NEP= primary production minus total respiration) and net ecosystem calcification (NEC=gross calcification minus gross CaCO3 dissolution) (Figure 1), and how these processes will change under global environmental change. The scientific objectives of this project were to investigate how biogeochemical processes and the relative contribution from NEP and NEC influence seawater CO2 chemistry in near-shore environments, using both controlled mesocosm experiments and in situ field observations. These objectives were integrated with educational components aimed at training undergraduate and graduate students in marine biogeochemical research, developing research driven teaching methods based on inquiry-, experience-, and collaborative-based learning, and working with the educational organization Ocean Discovery Institute (ODI) to engage individuals from an underrepresented community in science through educational activities and internships focused on ocean acidification (Figure 2). The educational activities of this project have contributed to the completion of ten graduate degrees (3 PhD, 5 MS, and 2 Master of Advanced Studies degrees), the participation of ~200 undergraduate students at UCSD in research driven instructional activities, and the involvement of ~100 ODI students (elementary to high school) investigating seawater acid-base chemistry in seagrass and tide pool environments. ODI concluded that our partnership served as an excellent model of collaboration between academia and a not-for-profit educational organization to engage students in active learning about ocean acidification and to envision themselves in STEM related careers. The success of this partnership was partly evident by the excellent student attendance, which exceeded 90%, afterschool, on a voluntary basis. The research activities of this project have contributed to more than twenty peer-reviewed publications relevant to the questions of the biogeochemical controls and variability of CO2 chemistry in the context of ocean acidification for a range of near-shore environments (e.g., coral reef, seagrass, salt marsh estuary, upwelling area). One of the key findings showed that different benthic communities indeed modify seawater chemistry differently according to the relative importance of NEP and NEC, which was also evident from total alkalinity-dissolved inorganic carbon (TA-DIC) relationships (Figure 3). This was in particular pronounced in experimental mesocosm experiments, which suggested that areas with high rates of daytime NEP complemented by increased CaCO3 dissolution at night could partially counteract increasing seawater acidity associated with anthropogenic OA at local scales. However, attempts to validate the mesocosm results across different benthic communities in natural coral reef environments were partly unsubstantiated as depth gradients and the biomass to water volume ratio had stronger influence on the seawater acid-base balance than the benthic community composition (Figure 4). Although the relative balance of NEP to NEC was still important, shallow reef habitats (<5 m) were so strongly dominated by NEP over diel timescales, that the average daily range of seawater acidity (measured as pH) could be reasonably well predicted from water column depth. This finding implies that local environmental variability regimes and potentially the vulnerability of reef organisms to ocean acidification, can potentially be predicted from widely available bathymetry data. Another important outcome of this project showed that the relative influence of NEP and NEC via TA-DIC analyses for different coral reef environments were dependent on the scale of the study. Over diel cycles, TA-DIC relationships were strongly driven by local benthic metabolism and community composition, but as the spatial scale increased, the TA-DIC relationship reflected processes that were integrated over larger spatiotemporal scales and multiple habitats, with effects of NEC becoming increasingly more dominant over NEP (Figure 5). Irrespective of this complexity, we concluded that TA anomalies relative to open ocean conditions offer useful insights to whether coral reef environments are net calcifying or dissolving, which will become an increasingly important property to monitor as global environmental conditions continue to change. A reoccurring outcome throughout the various research activities in this project showed clear evidence that CaCO3 dissolution will increase in response to ocean acidification, pushing carbonate reefs closer to a condition of net erosion. Increasing CaCO3 dissolution combined with forecasted changes in community composition and NEP could partially counteract ocean acidification by up to 25% by the end of this century in some reef environments (Figure 6). However, this level of buffering requires that reefs completely stop accreting CaCO3, which would imply an end to one of its most important ecosystem services. Last Modified: 08/31/2020 Submitted by: Andreas Andersson