The paper — Seaweed ecosystems may not mitigate CO2 emissions (Gallagher et al., 2022) — claims that seaweed ecosystems are carbon sources rather than carbon sinks because ‘respiration subsidies’ (from inputs of allochthonous organic carbon) create negative net ecosystem production. That is, that seaweed ecosystems produce more CO2 than they draw down, and thus may not mitigate CO2 emissions. They make this claim using a compiled dataset which shows that, on average, seaweed ecosystems are net heterotrophic. However, their assessment is flawed and conceptually misleading as key terms are misinterpreted, the data presented are biased, and the conclusions are not supported statistically. Here we discuss four flaws in the argument presented by Gallagher et al., which we believe risk confusing further research on seaweed blue carbon and unjustifiably seeding doubt around motivations and initiatives to protect and restore seaweed forests.
Abstract. The concentration of dissolved organic carbon (DOC) in groundwater emanating as spring discharge at several locations in Florida, USA, and the net rate of DOC increase in the downstream receiving waters were measured as part of a larger investigation of carbon dynamics in flowing waters. Springs with high discharge (>2.8 m3 s−1) were found to be the most organic-poor natural waters yet reported (13 ±1.6 μmol C L−1), while springs with lesser discharge exhibited somewhat higher DOC concentrations (values ranging from 30 to 77 μmol C L−1). DOC concentrations increased rapidly downstream from the point of spring discharge, with the calculated net areal input rate of DOC ranging from 0.04 to 1.64 mol C m−2 d−1 across springs. Rates of DOC increase were generally greater in those springs with high discharge rates. These input rates compare favorably with values reported for gross primary production in these macrophyte-dominated spring systems, assuming that 17% of macrophyte primary production is lost, on average, as DOC. The measures reported here are possible only because of the remarkably low DOC levels in the up-surging groundwaters and the short residency times of the water in the spring-runs themselves.
Empirical relationships between phytoplankton biomass and nutrient concentrations established across a wide range of different ecosystems constitute fundamental quantitative tools for predicting effects of nutrient management plans. Nutrient management plans based on such relationships, mostly established over trends of increasing rather than decreasing nutrient concentrations, assume full reversibility of coastal eutrophication. Monitoring data from 28 ecosystems located in four well-studied regions were analyzed to study the generality of chlorophyll a versus nutrient relationships and their applicability for ecosystem management. We demonstrate significant differences across regions as well as between specific coastal ecosystems within regions in the response of chlorophyll a to changing nitrogen concentrations. We also show that the chlorophyll a versus nitrogen relationships over time constitute convoluted trajectories rather than simple unique relationships. The ratio of chlorophyll a to total nitrogen almost doubled over the last 30-40 years across all regions. The uniformity of these trends, or shifting baselines, suggest they may result from large-scale changes, possibly associated with global climate change and increasing human stress on coastal ecosystems. Ecosystem management must, therefore, develop adaptation strategies to face shifting baselines and maintain ecosystem services at a sustainable level rather than striving to restore an ecosystem state of the past.
The pelagic realm of the dark ocean represents a key site for remineralization of organic matter and long‐term carbon storage and burial in the biosphere. It contains the largest pool of microbes in aquatic systems, harboring nearly 75% and 50% of the prokaryotic biomass and production, respectively, of the global ocean. Genomic approaches continue to uncover the enormous and dynamic genetic variability at phylogenetic and functional levels. Deep‐sea prokaryotes have comparable or even higher cell‐specific extracellular enzymatic activity than do microbes in surface waters, with a high fraction of freely released exoenzymes, probably indicative of a life mode reliant on surface attachment to particles or colloids. Additionally, evidence increases that chemoautotrophy might represent a significant CO 2 sink and source of primary production in the dark ocean. Recent advances challenge the paradigm of stable microbial food web structure and function and slow organic‐matter cycling. However, knowledge of deep‐ocean food webs is still rudimentary. Dynamics of particle transformation and fate of the exported material in deep waters are still largely unknown. Discrepancies exist between estimates of carbon fluxes and remineralization rates. Recent assessments, however, suggest that integrated respiration in the dark ocean's water column is comparable to that in the epipelagic zone, and that the dark ocean is a site of paramount importance for material cycling in the biosphere. The advent of new molecular tools and in situ sampling methodologies will improve knowledge of the dark ocean's microbial ecosystem and resolve current discrepancies between carbon sources and metabolic requirements of deep‐sea microbes.
This study quantified climate-driven changes and spatial variability in key environmental drivers over four decades along Greenland's coastal and shelf marine ecosystems and evaluated their impacts on marine biota divided into six regions. We analyzed trends in sea ice concentration and seasonality, sea surface temperatures, salinity, and freshwater inputs from ice discharge and freshwater runoff. West, East, and Southeast Greenland were most impacted by climate change, driven by increasing sea surface temperatures (0.22-0.5 °C decade-1), freshwater inputs (10.14-24.93 Gt yr-1 decade-1), declining sea ice concentrations (3-5.3 % decade-1), and more open water days (10.92-23.9 days decade-1). The Northwest and Northeast regions appeared more resilient due to lower sea surface temperature increases (0.01-0.03 °C decade-1) and sea ice declines (0.5-2.1 % decade-1). Changes in Southwest Greenland were limited to sea surface temperature (0.27 °C decade-1) and freshwater runoff (7.66 Gt yr-1 decade-1) increases since the 1990s. Synthesized evidence from 94 marine biota time series showed 73 exhibiting significant changes, and 37 identified an environmental driver: sea ice (20), temperature (19), and runoff (2). Only four time series considered multiple drivers. Biota time series trends mirrored regional environmental changes; 78 % changed significantly in West, East and Southeast regions combined, 73 % in southwest, and 56 % in the northern regions. Fish, benthic flora, and benthic fauna responses remained unclear due to data gaps, underscoring the need for further research. In conclusion, our findings reveal widespread biological change linked to climate but with distinct regional patterns in environmental drivers and associated responses across Greenland.
