Estimating Respiration Rates and Secondary Production of Macrobenthic Communities Across Coastal Habitats with Contrasting Structural Biodiversity

It's time to broaden what we consider a 'blue carbon ecosystem'

Photoautotrophic marine ecosystems can lock up organic carbon in their biomass and the associated organic sediments they trap over millennia and are thus regarded as blue carbon ecosystems. Because of the ability of marine ecosystems to lock up organic carbon for millennia, blue carbon is receiving much attention within the United Nations' 2030 Agenda for Sustainable Development as a nature-based solution (NBS) to climate change, but classically still focuses on seagrass meadows, mangrove forests, and tidal marshes. However, other coastal ecosystems could also be important for blue carbon storage, but remain largely neglected in both carbon cycling budgets and NBS strategic planning. Using a meta-analysis of 253 research publications, we identify other coastal ecosystems-including mud flats, fjords, coralline algal (rhodolith) beds, and some components or coral reef systems-with a strong capacity to act as blue carbon sinks in certain situations. Features that promote blue carbon burial within these 'non-classical' blue carbon ecosystems included: (1) balancing of carbon release by calcification via carbon uptake at the individual and ecosystem levels; (2) high rates of allochthonous organic carbon supply because of high particle trapping capacity; (3) high rates of carbon preservation and low remineralization rates; and (4) location in depositional environments. Some of these features are context-dependent, meaning that these ecosystems were blue carbon sinks in some locations, but not others. Therefore, we provide a universal framework that can evaluate the likelihood of a given ecosystem to behave as a blue carbon sink for a given context. Overall, this paper seeks to encourage consideration of non-classical blue carbon ecosystems within NBS strategies, allowing more complete blue carbon accounting.

Effects of a chronic impact on Cymodocea nodosa community carbon metabolism and dissolved organic carbon fluxes

Seagrass communities have been degraded worldwide experiencing elevated shoot density reduction by anthropogenic chronic pressures. This study aims to assess how a chronic (i.e., low intensity but long-lasting) impact that promotes reduced shoot density in a temperate seagrass population may affect community components and functioning. To this end, shoot density was reduced (0, 40, and 75 %) for three months in contrasting seasons (winter and summer), and assessed its effects on biotic components (i.e., seagrasses, macroalgae, macrofauna, and microphytobenthos), as well as on community carbon metabolism, dissolved organic carbon (DOC) fluxes and sediment organic matter (OM) content. Lower shoot densities enhanced the presence of macroalgae and microphytobenthos in the community, while macrofauna remained unchanged. Net community production was significantly reduced with the simulated reduction in shoot density in both seasons (up to 10-fold lower), which shifted the community in winter from being largely autotrophic (CO2 sink) to heterotrophic (CO2 source). This was due to the expected reduction in gross primary production, but also to the unexpected increase in community respiration (up to 2.2-fold higher). Since OM in the sediment was reduced in the simulated shoot density reduction treatments, the increase in sediment bacterial activity may help explain the increase in community respiration. DOC fluxes were also greatly reduced in both seasons (up to 5.5-fold lower), which coupled with the reduced net community production and loss of OM in the sediment may have a continued silent effect on blue carbon capture and storage capacity in this chronically stressed community. This study therefore highlights the importance of chronic impacts that promote the degradation of seagrass communities that may reduce their ability to provide highly valuable ecological services, including the ability to cope with the effects of climate change.

Deciphering benthic ecosystem functioning and resilience in a major port and marine protected area via the multi-trait approach

The preservation of ecosystem functioning of coastal zones, in face of increasing environmental stressors and species extinctions, relies on the functional redundancy and inherent resilience of its inhabitants. To compare the benthic functioning and resilience of a disturbed area with a relatively less impacted area, a study was conducted in Mumbai Port and Malvan Marine Protected Area (MPA), which exhibited contrasting characteristics. The hypothesis posited that the anthropogenically influenced Mumbai port would exhibit lower functional parameters and resilience compared to Malvan. Overall, the MPA presented higher species richness and functional diversity with a greater presence of sensitive species, while Mumbai was dominated by the presence of opportunistic species, as anticipated. However, our findings demonstrated that despite varied trends in species diversity metrics, in both the coastal areas, the resemblance in benthic functioning was high due to similarity in dominant trait profiles. Surprisingly, Functional Richness was higher at Mumbai, while Functional Evenness, Divergence and Dispersion were comparable at both sites. The resilience, as quantified by Functional Redundancy, was also comparable at both areas attributable to the presence of clusters of species with similar traits and a low occurrence of rare traits. The combination of traits observed in both areas was influenced by the extant environmental conditions, as revealed by RLQ analyses. This study underscores the valuable insights provided by the application of Biological Trait Analysis (BTA) tool in deciphering the relationship between species diversity and ecosystem functioning, as well as the resilience capabilities of ecosystems subjected to varying levels of perturbation. Moreover, the incorporation of functional diversity indices yielded valuable inferences regarding ecosystems resilience, which can aid future ecosystem management strategies.

Impacts of organic enrichment on macrobenthic production, productivity, and transfer efficiency: What can we learn from a gradient of sewage effluents?

We studied the macrobenthic invertebrate biomass (B), production (P), productivity (P/B̅-ratio), and transfer efficiency (TE) influenced by sewage effluents discharge in a diffusion zone. Our results indicated a clear distribution pattern of macrofauna communities along the sewage discharge gradient where biological factors (B, P, P/B̅, and TE) were driven by changes observed in community structure, composition, and the influence of environmental variables. The lowest B, P, and P/B̅ were observed at the stations sampled close to the pipelines. Abundance, biomass, production, and productivity increased with increasing distance from the pipelines toward stations placed at 100 m distance and then decreased toward the stations placed at >200 m, where there was a negative relationship between TE and B of macrofauna at sampling stations. Overall, there was a clear influence of the sewage discharge on macrofauna communities, but surrounding environment was influenced moderately by organic impact and discharges had no negative impacts.

Ecological habitat quality assessment of a tropical estuary using macrobenthic functional characteristics and biotic indices

Seasonal variation in the macrobenthic functional characteristics, such as trophic structure and secondary production, was studied, along with their structural characteristics such as density, biomass, and community distribution in Cochin estuary (CE), a tropical monsoonal estuary along the Southwest coast of India during 2018-2019. The biotic indices, AMBI (AZTI's Marine Biotic Index) and M-AMBI (Multivariate-AMBI) were used to assess the ecological quality of the study area by using the sensitivity of macrobenthic fauna to disturbances. A distinct temporal variation was evident in the macrobenthic structural characteristics, wherein high density, biomass, and species richness were observed during the post-monsoon. Polycheate species were dominant during pre-monsoon (Prionospio cirrifera) and monsoon (Mediomastus sp.), while gastropod species, Stenothyra perpumila, was dominant during post-monsoon. The trophic structure analysis revealed the dominance of deposit-feeding polychaetes during the pre-monsoon in association with the fine sediments laden with high organic carbon. In contrast, during post-monsoon, the polychaete trophic groups were more or less evenly distributed in the sandy substratum. The higher density of suspension feeders and herbivorous-grazers observed during the post-monsoon indicated improved habitat quality compared to the pre-monsoon dominated by the detritivores. The macrobenthic secondary production was also high during the post-monsoon, contributed by a diverse assemblage of molluscs, crustaceans, and polychaetes having size > 0.7 mm. The biotic indices also showed an evident increase in the habitat quality of the estuary from pre-monsoon to post-monsoon (AMBI, moderately disturbed to undisturbed; M-AMBI, poor-moderate-good to good-high conditions), similar to the results of benthic structural and functional characteristics. The study highlights the significance of the utility of benthic functional characteristics while assessing the habitat quality of an ecosystem.

Positive contribution of macrofaunal biodiversity to secondary production and seagrass carbon metabolism

Coastal vegetated habitats such as seagrasses are known to play a critical role in carbon cycling, and their potential to mitigate climate change as blue carbon habitats have been repeatedly highlighted. However, little is known about the role of associated macrofauna communities on the dynamics of critical processes of seagrass carbon metabolism (e.g. respiration, turnover, and production). We conducted a field study across a spatial gradient of seagrass meadows involving variable environmental conditions and macrobenthic diversity to investigate (1) the relationship between macrofauna biodiversity and secondary production (i.e. consumer incorporation of organic matter per time unit), and (2) the role of macrofauna communities in seagrass organic carbon metabolism (i.e. respiration and primary production). We show that while several environmental factors influence secondary production, macrofauna biodiversity controls the range of local seagrass secondary production. We demonstrate that macrofauna respiration rates were responsible for almost 40 % of the overall seafloor community respiration. Macrofauna represented on average > 25% of the total benthic organic C stocks, high secondary production that likely becomes available to upper trophic levels of the coastal food web. Our findings support the role of macrofauna biodiversity in maintaining productive ecosystems, implying that biodiversity loss due to ongoing environmental change yields less productive seagrass ecosystems. Hence, the assessment of carbon dynamics in coastal habitats should include associated macrofauna biodiversity elements if we aim to obtain robust estimates of global carbon budgets required to implement management actions for the sustainable functioning of the worlds' coasts. This article is protected by copyright. All rights reserved.

Loss of benthic macrofauna functional traits correlates with changes in sediment biogeochemistry along an extreme salinity gradient in the Coorong lagoon, Australia

Estuarine ecosystems are considered hotspots for productivity, biogeochemical cycling and biodiversity, however, their functions and services are threatened by several anthropogenic pressures. We investigated how abundance and diversity of benthic macrofauna, and their functional traits, correlate to sediment biogeochemistry and nutrient concentrations throughout an estuarine-to-hypersaline lagoon. Benthic communities and functional traits were significantly different across the sites analysed, with higher abundance and more traits expressed in the estuarine region. The results revealed that the benthic trait differences correlated with sediment biogeochemistry and nutrient concentrations in the system. The estuarine regions were dominated by high abundance of large burrowing and bioturbating macrofauna, promoting nutrient cycling and organic matter mineralisation, while these organisms were absent in the hypersaline lagoon, favouring accumulation of organic matter and nutrients in the sediment. The results highlight the importance of preserving healthy benthic communities to maintain ecosystem functioning and mitigate the potential impacts of eutrophication in estuarine ecosystems.

Food-web comparisons between two shallow vegetated habitat types in the Baltic Sea

Coastal vegetated habitats maintain highly diverse communities, where the contribution of macrophyte production is significant for macroinvertebrate primary consumers. In the brackish-waters of the Baltic Sea, the taxonomical diversity of different macrophytes includes both marine and limnic species. To study the basal food-web differences of two key vegetated habitat types, either dominated by a perennial brown macroalgae (Fucus vesiculosus) or by angiosperm plants, ¹³C and ¹⁵N compositions of different primary producers and macroinvertebrate consumers were examined, and their diets were estimated by Bayesian mixing models. Carbon isotope diversity of primary producers was high especially in the hard-bottom Fucus-dominated habitats, which was also reflected in a larger consumer isotope niche. However, consumer isotope niche among sites was similar within the same habitat type. Our models indicated that the perennial macrophyte dietary median contribution was about 25% for deposit feeders and omnivores in both habitat types, while epigrazers preferred filamentous algae (30-60%). The niche positions of the abundant clams L. balthica, M. arenaria and C. glaucum differed between the two habitats, but they showed only small (<10% units) differences in their macrophyte dietary contributions. The isotopic compositions of the dominating primary producer assemblage reflected significantly in the isotope niche structure of the associated primary consumers.

Variable contributions of seafloor communities to ecosystem metabolism across a gradient of habitat-forming species

The contributions of habitat-forming species to the biodiversity and ecosystem processes of marine and terrestrial ecosystems are widely recognized. Aquatic plants are considered foundation species in shallow ecosystems, as they maintain biodiversity and sustain many ecosystem functions such as primary production and respiration. Despite the increasing amount of biodiversity-ecosystem functioning experiments in seagrass habitats, the effects of benthic variability on ecosystem functioning are rarely investigated across spatially variable aquatic plant habitats. Here, we quantitatively link seasonal variability in seafloor metabolism (i.e. gross primary production and community respiration) with major benthic community components (i.e. microphytobenthos, aquatic plants and macrofauna) across a structural complexity gradient of habitat-forming species (in terms of shoot density and biomass), ranging from bare sand, to a sparse mixture of plants to a dense monospecific seagrass meadow. The increasing complexity gradient enhanced the magnitude of the relationships between benthic community and seafloor metabolism. The daily average seafloor metabolism per season at the bare site was similar to the sparse site, highlighting the role of microphytobenthos for seafloor metabolism in shallow unvegetated sediments. The contribution of the associated macrofauna to the seafloor respiration was similar to the aquatic plant community contribution. Infauna was the main macrofaunal component significantly explaining the seasonal variability of seafloor respiration. However, benthic community-metabolism relationships were stronger within the plant community than within the macrofauna community (i.e. steepest slopes and lowest p-values). Understanding these relationships are a priority since climate change and biodiversity loss are reducing habitat complexity around the world, jeopardizing valuable ecosystem functions and services.

Detrimental impacts of climate change may be exacerbated by density-dependent population regulation in blue mussels

The climate on our planet is changing and the range distributions of organisms are shifting in response. In aquatic environments, species might not be able to redistribute poleward or into deeper water when temperatures rise because of barriers, reduced light availability, altered water chemistry or any combination of these. How species respond to climate change may depend on physiological adaptability, but also on the population dynamics of the species. Density dependence is a ubiquitous force that governs population dynamics and regulates population growth, yet its connections to the impacts of climate change remain little known, especially in marine studies. Reductions in density below an environmental carrying capacity may cause compensatory increases in demographic parameters and population growth rate, hence masking the impacts of climate change on populations. On the other hand, climate-driven deterioration of conditions may reduce environmental carrying capacities, making compensation less likely and populations more susceptible to the effects of stochastic processes. Here we investigate the effects of climate change on Baltic blue mussels using a 17-year dataset on population density. Using a Bayesian modelling framework, we investigate the impacts of climate change, assess the magnitude and effects of density dependence, and project the likelihood of population decline by the year 2030. Our findings show negative impacts of warmer and less saline waters, both outcomes of climate change. We also show that density dependence increases the likelihood of population decline by subjecting the population to the detrimental effects of stochastic processes (i.e. low densities where random bad years can cause local extinction, negating the possibility for random good years to offset bad years). We highlight the importance of understanding, and accounting for both density dependence and climate variation when predicting the impact of climate change on keystone species, such as the Baltic blue mussel.

The meagre future of benthic fauna in a coastal sea-Benthic responses to recovery from eutrophication in a changing climate

Nutrient loading and climate change affect coastal ecosystems worldwide. Unravelling the combined effects of these pressures on benthic macrofauna is essential for understanding the future functioning of coastal ecosystems, as it is an important component linking the benthic and pelagic realms. In this study, we extended an existing model of benthic macrofauna coupled with a physical-biogeochemical model of the Baltic Sea to study the combined effects of changing nutrient loads and climate on biomass and metabolism of benthic macrofauna historically and in scenarios for the future. Based on a statistical comparison with a large validation dataset of measured biomasses, the model showed good or reasonable performance across the different basins and depth strata in the model area. In scenarios with decreasing nutrient loads according to the Baltic Sea Action Plan but also with continued recent loads (mean loads 2012-2014), overall macrofaunal biomass and carbon processing were projected to decrease significantly by the end of the century despite improved oxygen conditions at the seafloor. Climate change led to intensified pelagic recycling of primary production and reduced export of particulate organic carbon to the seafloor with negative effects on macrofaunal biomass. In the high nutrient load scenario, representing the highest recorded historical loads, climate change counteracted the effects of increased productivity leading to a hyperbolic response: biomass and carbon processing increased up to mid-21st century but then decreased, giving almost no net change by the end of the 21st century compared to present. The study shows that benthic responses to environmental change are nonlinear and partly decoupled from pelagic responses and indicates that benthic-pelagic coupling might be weaker in a warmer and less eutrophic sea.