Seagrass ecosystems as a globally significant carbon stock

The protection of organic carbon stored in forests is considered as an important method for mitigating climate change. Like terrestrial ecosystems, coastal ecosystems store large amounts of carbon, and there are initiatives to protect these ‘blue carbon’ stores. Organic carbon stocks in tidal salt marshes and mangroves have been estimated, but uncertainties in the stores of seagrass meadows—some of the most productive ecosystems on Earth—hinder the application of marine carbon conservation schemes. Here, we compile published and unpublished measurements of the organic carbon content of living seagrass biomass and underlying soils in 946 distinct seagrass meadows across the globe. Using only data from sites for which full inventories exist, we estimate that, globally, seagrass ecosystems could store as much as 19.9 Pg organic carbon; according to a more conservative approach, in which we incorporate more data from surface soils and depth-dependent declines in soil carbon stocks, we estimate that the seagrass carbon pool lies between 4.2 and 8.4 Pg carbon. We estimate that present rates of seagrass loss could result in the release of up to 299 Tg carbon per year, assuming that all of the organic carbon in seagrass biomass and the top metre of soils is remineralized.

Controls on Ecosystem Carbon Dioxide Exchange in Short- and Long-Hydroperiod Florida Everglades Freshwater Marshes

Although freshwater wetlands are among the most productive ecosystems on Earth, little is known of carbon dioxide (CO2) exchange in low latitude wetlands. The Everglades is an extensive, oligotrophic wetland in south Florida characterized by short- and long-hydroperiod marshes. Chamber-based CO2 exchange measurements were made to compare the marshes and examine the roles of primary producers, seasonality, and environmental drivers in determining exchange rates. Low rates of CO2 exchange were observed in both marshes with net ecosystem production reaching maxima of 3.77 and 4.28 μmol CO2 m−2 s−1 in short- and long-hydroperiod marshes, respectively. Fluxes of CO2 were affected by seasonality only in the short-hydroperiod marsh, where flux rates were significantly lower in the wet season than in the dry season. Emergent macrophytes dominated fluxes at both sites, though this was not the case for the short-hydroperiod marsh in the wet season. Water depth, a factor partly under human control, significantly affected gross ecosystem production at the short-hydroperiod marsh. As Everglades ecosystem restoration proceeds, leading to deeper water and longer hydroperiods, productivity in short-hydroperiod marshes will likely be more negatively affected than in long-hydroperiod marshes. The Everglades stand in contrast to many freshwater wetlands because of ecosystem-wide low productivity rates.

Seasonal differences in the CO2 exchange of a short-hydroperiod Florida Everglades marsh

Although wetlands are among the world's most productive ecosystems, little is known of long-term CO2 exchange in tropical and subtropical wetlands. The Everglades is a highly managed wetlands complex occupying >6000 km2 in south Florida. This ecosystem is oligotrophic, but extremely high rates of productivity have been previously reported. To evaluate CO2 exchange and its response to seasonality (dry vs. wet season) in the Everglades, an eddy covariance tower was established in a short-hydroperiod marl marsh. Rates of net ecosystem exchange and ecosystem respiration were small year-round and declined in the wet season relative to the dry season. Inundation reduced macrophyte CO2 uptake, substantially limiting gross ecosystem production. While light and air temperature exerted the primary controls on net ecosystem exchange and ecosystem respiration in the dry season, inundation weakened these relationships. The ecosystem shifted from a CO2 sink in the dry season to a CO2 source in the wet season; however, the marsh was a small carbon sink on an annual basis. Net ecosystem production, ecosystem respiration, and gross ecosystem production were −49.9, 446.1 and 496.0 g C m−2 year−1, respectively. Unexpectedly low CO2 flux rates and annual production distinguish the Everglades from many other wetlands. Nonetheless, impending changes in water management are likely to alter the CO2 balance of this wetland and may increase the source strength of these extensive short-hydroperiod wetlands.

Density-Dependent Effects of Coral Reef Restoration on Herbivory and Community Interactions

Coral reefs are among the most productive ecosystems in the world. Yet, with their recent declines due to disease, climate change, and overfishing, restoration of these habitats is one of the main concerns for ecologists, resource managers, and government organizations. Coral reef restoration aims to promote key ecosystem processes to shift these habitats to their historical state of high coral cover, but few studies have focused on effective ways to promote resilience. In addition, little is known about the impact of restoration on the fish communities. The aim of this study is to understand how the community of herbivorous fishes is affected by the density of coral outplants inside a special protection area located in the Florida Keys National Marine Sanctuary. Grazing rates, number of visits and time spent foraging were compared using video footage of sites previously devoid of corals, and six months after coral restorations had occurred. Coral transplantations did not appear to attract herbivores nor increase grazing rates of fishes. Instead Sparisoma and Acanthurus fishes appear to respond to changes in the environment by modifying their grazing behavior. However, there was an observed increase in visits by Acanthurus species after transplantation for all the sites sampled within the reef. These fishes seemed to prefer low coral cover sites for grazing. This study highlights the importance of examining coral restorations impacts at the community level. Understanding how restoration influences herbivores and other guilds of reef fishes will allow individuals to not only determine if these habitats are returning to their “original” state, but provide more information on the ways these systems cope with changes in the environment.

Spatial scale and abundance patterns of large fish communities in freshwater marshes of the Florida Everglades

Anthropogenic habitat alterations and water-management practices have imposed an artificial spatial scale onto the once contiguous freshwater marshes of the Florida Everglades. To gain insight into how these changes may affect biotic communities, we examined whether variation in the abundance and community structure of large fishes (SL . 8 cm) in Everglades marshes varied more at regional or intraregional scales, and whether this variation was related to hydroperiod, water depth, floating mat volume, and vegetation density. From October 1997 to October 2002, we used an airboat electrofisher to sample large fishes at sites within three regions of the Everglades. Each of these regions is subject to unique watermanagement schedules. Dry-down events (water depth , 10 cm) occurred at several sites during spring in 1999, 2000, 2001, and 2002. The 2001 dry-down event was the most severe and widespread. Abundance of several fishes decreased significantly through time, and the number of days post-dry-down covaried significantly with abundance for several species. Processes operating at the regional scale appear to play important roles in regulating large fishes. The most pronounced patterns in abundance and community structure occurred at the regional scale, and the effect size for region was greater than the effect size for sites nested within region for abundance of all species combined, all predators combined, and each of the seven most abundant species. Non-metric multi-dimensional scaling revealed distinct groupings of sites corresponding to the three regions. We also found significant variation in community structure through time that correlated with the number of days post-dry-down. Our results suggest that hydroperiod and water management at the regional scale influence large fish communities of Everglades marshes.