] The San Joaquin Freshwater Marsh (SJFM) is a seasonally flooded Typha wetland in Southern California that is characterized by high rates of Aboveground Net Primary Production (ANPP) and a large accumulation of standing leaf litter. The ANPP, Gross and Net Ecosystem CO₂ Exchange (GEE and NEE), and Enhanced Vegetation Index (EVI) at the SJFM fluctuate by ∼40% from year to year, in ways that are not directly attributable to variation in weather or the maximum green Leaf Area Index (LAI_max). We tested the hypothesis that this variation is caused by a negative feedback between ANPP, the buildup of leaf litter, shading of green leaves by litter, a reduction in GEE and NEE, and a subsequent reduction in ANPP. Litter manipulations on replicated plots demonstrated that the presence of standing litter decreased plot-level NEE by 17 to 47% and surface EVI by 25 to 48%, even as green Leaf Area Index (LAI_green) was held constant. Plot level NEE and surface EVI remained tightly correlated, and largely decoupled from LAI_green, as standing litter was varied. This pattern paralleled that observed for the entire marsh, where NEE and EVI remained tightly correlated, and largely decoupled from LAI_max, from year to year. Correcting LAI_green and LAI_max for the amount of shading caused by standing litter improved the correlations between LAI and EVI and NEE, indicating that EVI and NEE are most sensitive to the amount of unshaded LAI. The accumulation of standing litter at the SJFM decouples the relationships between LAI and EVI and NEE, and appears to be important for controlling the interannual variability observed at the site.

Rising atmospheric CO 2 and its equilibration with surface ocean seawater is lowering both the pH and carbonate saturation state (Ω) of the oceans. Numerous calcifying organisms, including reef-building corals, may be severely impacted by declining aragonite and calcite saturation, but the fate of coral reef ecosystems in response to ocean acidification remains largely unexplored. Naturally low saturation (Ω ~ 0.5) low pH (6.70-7.30) groundwater has been discharging for millennia at localized submarine springs (called "ojos") at Puerto Morelos, México near the Mesoamerican Reef. This ecosystem provides insights into potential long term responses of coral ecosystems to low saturation conditions. In-situ chemical and biological data indicate that both coral species richness and coral colony size decline with increasing proximity to low-saturation, low-pH waters at the ojo centers. Only three scleractinian coral species (Porites astreoides, Porites divaricata, and Siderastrea radians) occur in undersaturated waters at all ojos examined. Because these three species are rarely major contributors to Caribbean reef framework, these data may indicate that today's more complex frame-building species may be replaced by smaller, possibly patchy, colonies of only a few species along the Mesoamerican Barrier Reef. The growth of these scleractinian coral species at undersaturated conditions illustrates that the response to ocean acidification is likely to vary across species and environments; thus, our data emphasize the need to better understand the mechanisms of calcification to more accurately predict future impacts of ocean acidification. © 2011 The Author(s).