There is consensus that temperature plays a major role in shaping microbial activity, but there are still questions as to how temperature influences different aspects of bacterioplankton carbon metabolism under different environmental conditions. We examined the temperature dependence of bacterioplankton carbon metabolism, whether this temperature dependence changes at different temperatures, and whether the relationship between temperature and carbon metabolism varies among estuarine sub-systems differing in their degree of enrichment. Two years of intensive sampling in a temperate estuary (Monie Bay, Chesapeake Bay, USA) revealed significant differences in the temperature dependence of bacterial production (BP) and respiration (BR), which drove a strong negative temperature response of bacterial growth efficiency (BGE). Accordingly, BGE was lower in summer (< 0.2) and higher in winter (> 0.5). For all measured metabolic processes, the most pronounced temperature response was observed at lower temperatures, with Q 10 values generally 2-fold greater than in warmer waters. Despite significant differences in resource availability, both the temperature dependence and magnitude of BR and bacterioplankton carbon consumption (BCC) were remarkably similar among the 4 estuarine sub-systems. Although temperature dependencies of BP and BGE were also similar, their magnitude differed significantly, with highest values in the nutrient-enriched sub-system and lowest in the open bay. This pattern in carbon metabolism among subsystems was present throughout the year and was confirmed by temperature manipulation experiments, suggesting the temperature effects on BP and BGE did not override the influence of resource availability. We conclude that temperature is the dominant factor regulating seasonality of BR and BCC in this system, whereas BP and BGE are influenced by both temperature and organic matter quality, with variation in the relative importance of each of these factors throughout the year. KEY WORDS: Bacterioplankton · Temperature · Growth efficiency · EstuaryResale or republication not permitted without written consent of the publisher

Bacterioplankton communities play a key role in aquatic carbon cycling, specifically with respect to the magnitude of organic carbon processed and partitioning of this carbon into biomass and respiratory losses. Studies of bacterioplankton carbon demand (BCD) and growth efficiency (BGE) frequently report higher values in more productive systems, suggesting these aspects of carbon metabolism may be positively coupled. However, the existence of such a relationship in natural aquatic systems has yet to be identified. Using a comprehensive 2-year study of bacterioplankton carbon metabolism in a temperate estuary, we investigated BCD and BGE and explored factors that may modulate their magnitude and coherence, including nutrient concentrations, dissolved nutrient uptake and source and quality of dissolved organic carbon (DOC). During the course of our study, BCD ranged from 0.4 to 15.9 lg l À1 h -1, with an overall mean of 3.8 lg l À1 h -1. Mean BGE was similar to that reported for other estuarine systems (0.32) and of comparable range (that is, 0.06-0.68). Initial analyses identified a negative correlation between BCD and BGE, yet removal of the effect of temperature revealed an underlying positive coupling that was also correlated with long-term DOC lability. Whereas BCD was weakly related to ambient DOC concentrations, neither BCD nor BGE showed any relationship with ambient nutrient concentrations or nutrient uptake stoichiometries. We conclude that in this carbon-rich estuary, organic matter source and quality play an important role in regulating the magnitude of carbon metabolism and may be more important than nutrient availability alone in the regulation of BGE.

Grazing mortality of the marine phytoplankton Synechococcus is dominated by planktonic protists, yet rates of consumption and factors regulating grazer-Synechococcus interactions are poorly understood. One aspect of predator-prey interactions for which little is known are the mechanisms by which Synechococcus avoids or resists predation and, in turn, how this relates to the ability of Synechococcus to support growth of protist grazer populations. Grazing experiments conducted with the raptorial dinoflagellate Oxyrrhis marina and phylogenetically diverse Synechococcus isolates (strains WH8102, CC9605, CC9311, and CC9902) revealed marked differences in grazing rates-specifically that WH8102 was grazed at significantly lower rates than all other isolates. Additional experiments using the heterotrophic nanoflagellate Goniomonas pacifica and the filter-feeding tintinnid ciliate Eutintinnis sp. revealed that this pattern in grazing susceptibility among the isolates transcended feeding guilds and grazer taxon. Synechococcus cell size, elemental ratios, and motility were not able to explain differences in grazing rates, indicating that other features play a primary role in grazing resistance. Growth of heterotrophic protists was poorly coupled to prey ingestion and was influenced by the strain of Synechococcus being consumed. Although Synechococcus was generally a poor-quality food source, it tended to support higher growth and survival of G. pacifica and O. marina relative to Eutintinnis sp., indicating that suitability of Synechococcus varies among grazer taxa and may be a more suitable food source for the smaller protist grazers. This work has developed tractable model systems for further studies of grazer-Synechococcus interactions in marine microbial food webs.

There is increasing urgency to implement climate change mitigation strategies that enhance greenhouse gas removal from the atmosphere and reduce carbon dioxide (CO2) emissions. Recently, coastal “blue carbon” habitats⁠—mangroves, salt marshes, and seagrass meadows—have received attention for their ability to capture CO2 and store organic carbon (OC), primarily in their sediments. Across habitat types and regions, however, information about the sequestration rates and sources of carbon to local sediments remains sparse. Here we compiled recently obtained estimates of sediment OC stocks and sequestration rates from 139 cores collected from temperate seagrass (Zostera marina) meadows in Alaska, British Columbia, Washington, and Oregon. Across all cores sediment OC content averaged 0.75%. Organic carbon stocks in the top 25 cm and 1 m of the sediment averaged 1,846 and 7,168 g OC m−2, respectively. Carbon sequestration rates ranged from 4.6 to 93.0 g OC m−2 yr−1 and averaged 24.8 g OC m−2 yr−1. Isotopic data from this region suggest that OC in the sediments is largely from noneelgrass sources. In general, these values are comparable to those from other temperate Z. marina meadows, but significantly lower than previously reported values for seagrasses globally. These results further highlight the need for local and species‐level quantification of blue carbon parameters. While temperate eelgrass meadows may not sequester and store as much carbon as seagrass meadows elsewhere, climate policy incentives should still be implemented to protect existing sediment carbon stocks and the other critical ecosystem services associated with eelgrass habitats.

The coastal ecosystems of temperate North America provide a variety of ecosystem services including high rates of carbon sequestration. Yet, little data exist for the carbon stocks of major tidal wetland types in the Pacific Northwest, United States. We quantified the total ecosystem carbon stocks (TECS) in seagrass, emergent marshes, and forested tidal wetlands, occurring along increasing elevation and decreasing salinity gradients. The TECS included the total aboveground carbon stocks and the entire soil profile (to as deep as 3 m). TECS significantly increased along the elevation and salinity gradients: 217 ± 60 Mg C/ha for seagrass (low elevation/high salinity), 417 ± 70 Mg C/ha for low marsh, 551 ± 47 Mg C/ha for high marsh, and 1,064 ± 38 Mg C/ha for tidal forest (high elevation/low salinity). Soil carbon stocks accounted for >98% of TECS in the seagrass and marsh communities and 78% in the tidal forest. Soils in the 0–100 cm portion of the profile accounted for only 48%–53% of the TECS in seagrasses and marshes and 34% of the TECS in tidal forests. Thus, the commonly applied limit defining TECS to a 100 cm depth would greatly underestimate both carbon stocks and potential greenhouse gas emissions from land‐use conversion. The large carbon stocks coupled with other ecosystem services suggest value in the conservation and restoration of temperate zone tidal wetlands through climate change mitigation strategies. However, the findings suggest that long‐term sea‐level rise effects such as tidal inundation and increased porewater salinity will likely decrease ecosystem carbon stocks in the absence of upslope wetland migration buffer zones.