Detecting Subtle Shifts in Ecosystem Functioning in a Dynamic Estuarine Environment

NZTD - The New Zealand Trait Database for shallow-water marine benthic invertebrates

Macrobenthic traits, for example feeding mode, life history, morphology, are increasingly used for determining responses of macrobenthic fauna to environmental change and influences on ecosystem functioning. Yet, trait information is scarce or non-existent in several parts of the world, such as New Zealand. This deficit makes collecting trait data a difficult and time-consuming task, limiting its potential use in trait-based assessments. Here, we present the New Zealand Trait Database (NZTD) for marine benthic invertebrates, the first comprehensive assessment of macrobenthic traits in New Zealand. The NZTD provides trait information for more than 700 macrobenthic taxa, categorised by 18 traits and 77 trait modalities. The NZTD includes five freely downloadable datasets, (1) the macrobenthic trait dataset, with outcomes from a fuzzy coding procedure, (2) the trait source information, (3) the references by taxa, (4) the full references list, and (5) the full taxa list used in the NZTD. Establishing the NZTD closes the trait knowledge gap in New Zealand and facilitates future research applying trait-based approaches to New Zealand's coastal macrofauna.

Real-world impacts of microplastic pollution on seafloor ecosystem function

Emerging research shows that microplastic pollution could be impacting seafloor ecosystem function, but this has been primarily demonstrated without environmental and ecological context. This causes uncertainty in the real-world effects of microplastic pollution and leaves out essential information guiding policy and mitigation. In this study, we take a well-supported sampling design and statistical approach commonly employed in benthic ecology to evaluate real-world effects of microplastic pollution on coastal, benthic ecosystem function. We utilised environmental gradients in the Waitemata Harbour of Auckland, New Zealand to evaluate the importance of commonly assessed biological, chemical, and geological sediment variables and the characteristics of microplastic contaminants in driving essential ecosystem functions. Our results showed that models including microplastic terms were more accurate and explained more variability than those without microplastic terms, highlighting that microplastics impact real-world seafloor ecosystem function. Specifically, microplastic fibers significantly influenced oxygen flux (p < 0.03) and the diverse forms of microplastics (i.e., richness) significantly influenced ammonium flux (p < 0.02). Additionally, interactions between microplastic fiber concentrations and mollusc abundances significantly contributed to oxygen flux (p < 0.02). These results provide the first evaluation of in situ relationships between microplastics and ecosystem function. Even more importantly, this study suggests the value of environmental and ecological context for addressing microplastic impacts on benthic ecosystems and argues for further field examination.

Seasonal variability in ecosystem functioning across estuarine gradients: The role of sediment communities and ecosystem processes

Functional trait approaches advance the understanding of biodiversity-ecosystem function (BDEF) relationships and its control by the environmental context. Application of these insights into management remains constrained due to lack of evidence from real-world ecosystems that capture the natural spatial and temporal gradients at which biodiversity and environmental conditions operate. In this study we measured macrofauna community traits, ecosystem processes and abiotic properties at 9 locations during 4 months, spanning a wide gradient in sedimentary habitats and salinity in the Scheldt estuary, and quantified the (a)biotic contribution to sediment community oxygen consumption, as a measure of ecosystem function. We found that functional attributes of the macrofauna community and its effect on bio-irrigation can predict ecosystem function, but especially during the colder period of the year. This result highlights that generalizations about BDEF relationships, and biodiversity loss on ecosystem functions, are limited whenever this temporal component is not acknowledged.

Large Infaunal Bivalves Determine Community Uptake of Macroalgal Detritus and Food Web Pathways

Abstract

Human activities alter biodiversity, influencing bottom-up and top-down control on food webs which can affect ecosystem functioning. In marine ecosystems, large bivalves play a critical role in benthic–pelagic coupling including nutrient cycling; however, their influence on the uptake of detrital organic matter by benthic communities is less understood. In a replicated factorial field experiment, we examined how the presence or absence (overharvesting scenario) of a large suspension-feeding clam on an intertidal sandflat and the addition of isotopically enriched macroalgal (Ulva sp.) detritus (eutrophication scenario) influenced infaunal biodiversity, and how changes in trophic interactions influenced key ecosystem functions (nutrient cycling and benthic metabolism and primary production). Both clams and Ulva increased community metabolism, but only clams had an effect on nutrient regeneration. We used the 13C- and 15N-enriched Ulva to quantify the effect of clams on detritus uptake in fauna and recovery in sediment. Due to their large biomass, nitrogen incorporation by clams constituted one-third of the infaunal community uptake after 14 days. Clam uptake likely resulted from ingestion of resuspended microphytobentos which had utilized 15N leaking out from decomposing Ulva. In plots without Ulva addition, the effect of clams on the overall resource utilization by the benthic community using natural abundance isotope niche metrics were tested. In plots without clams, the isotope niche of the community was reduced, and less carbon of pelagic origin was channelled into the infaunal food web. Our results imply that the loss of clams changes trophic pathways and reduces community uptake of macroalgal detritus, potentially exacerbating eutrophication.

Graphic Abstract

Spatial and temporal variability of carbon budgets of shallow South African subtropical estuaries

Estuarine carbon fluxes constitute a significant component of coastal CO₂ emissions and nutrients recycling, but high uncertainty is still present due to the heterogeneity of these areas. Although South Africa has nearly 300 estuaries, very little is known about their contribution to carbon emissions or sequestration. This study aims to provide a first estimation of the carbon emissions and nutrient fluxes of South African sub-tropical estuaries through a direct quantification of respiration, primary production and nutrient regeneration of benthic and planktonic communities. In order to account for the extreme variability in subtropical estuarine areas, due to seasonality in rainfall, two estuaries with opposite characteristics were studied; the temporarily open/closed Mdloti Estuary subjected to strong anthropic pressure, and the permanently open Mlalazi Estuary located in a natural reserve. Field deployment of benthic chambers and clear/dark bottles assessed oxygen, ammonia and phosphate fluxes of both benthic and planktonic communities. An inverse pattern between benthic and pelagic primary production was found in both estuaries. Different drivers related to mouth status and sediment characteristics were identified in the two estuaries. The annual average carbon emission indicates that the two systems are heterotrophic over the year releasing substantial CO₂ emissions into the atmosphere. Results show that carbon fluxes in subtropical estuaries are extremely variable in space and time. Future up-scaling carbon estimations need to account for those small scale and regional dynamics.

Sediment carbon and nutrient fluxes from cleared and intact temperate mangrove ecosystems and adjacent sandflats

The loss of mangrove ecosystems is associated with numerous impacts on coastal and estuarine function, including sediment carbon and nutrient cycling. In this study we compared in situ fluxes of carbon dioxide (CO₂) from the sediment to the atmosphere, and fluxes of dissolved inorganic nutrients and oxygen across the sediment-water interface, in intact and cleared mangrove and sandflat ecosystems in a temperate estuary. Measurements were made 20 and 25months after mangrove clearance, in summer and winter, respectively. Sediment CO₂ efflux was over two-fold higher from cleared than intact mangrove ecosystems at 20 and 25months after mangrove clearance. The higher CO₂ efflux from the cleared site was explained by an increase in respiration of dead root material along with sediment disturbance following mangrove clearance. In contrast, sediment CO₂ efflux from the sandflat site was negligible (≤9.13±1.18mmolm^-2d^-1), associated with lower sediment organic matter content. The fluxes of inorganic nutrients (NH₄⁺, NO_x and PO₄^3-) from intact and cleared mangrove sediments were low (≤20.37±18.66μmolm^-2h^-1). The highest NH₄⁺ fluxes were measured at the sandflat site (69.21±13.49μmolm^-2h^-1). Lower inorganic nutrient fluxes within the cleared and intact mangrove sites compared to the sandflat site were associated with lower abundance of larger burrowing macrofauna. Further, a higher fraction of organic matter, silt and clay content in mangrove sediments may have limited nutrient exchange.

Global meta-analysis of native and nonindigenous trophic traits in aquatic ecosystems

Ecologists have recently devoted their attention to the study of species traits and their role in the establishment and spread of nonindigenous species (NIS). However, research efforts have mostly focused on studies of terrestrial taxa, with lesser attention being dedicated to aquatic species. Aquatic habitats comprise of interconnected waterways, as well as exclusive introduction vectors that allow unparalleled artificial transport of species and their propagules. Consequently, species traits that commonly facilitate biological invasions in terrestrial systems may not be as represented in aquatic environments. We provide a global meta-analysis of studies conducted in both marine and freshwater habitats. We selected studies that conducted experiments with native and NIS under common environmental conditions to allow detailed comparisons among species traits. In addition, we explored whether different factors such as species relatedness, functional feeding groups, latitude, climate, and experimental conditions could be linked to predictive traits. Our results show that species with traits that enhance consumption and growth have a substantially increased probability of establishing and spreading when entering novel ecosystems. Moreover, traits associated with predatory avoidance were more prevalent in NIS and therefore favour invasive species in aquatic habitats. When we analysed NIS interacting with taxonomically distinctive native taxa, we found that consumption and growth were particularly important traits. This suggests that particular attention should be paid to newly introduced species for which there are no close relatives in the local biota. Finally, we found a bias towards studies conducted in temperate regions, and thus, more studies in other climatic regions are needed. We conclude that studies aiming at predicting future range shifts should consider trophic traits of aquatic NIS as these traits are indicative of multiple interacting mechanisms involved in promoting species invasions.

In situ soft sediment nutrient enrichment: A unified approach to eutrophication field experiments

Adding fertiliser to sediments is an established way of studying the effects of eutrophication but a lack of consistent methodology, reporting on enrichment levels, or guidance on application rates precludes rigorous synthesis and meta-analysis. We developed a simple enrichment technique then applied it to 28 sites across an intertidal sandflat. Fertiliser application rates of 150 and 600gNm(-2) resulted in pore water ammonium concentrations respectively 1-110 and 4-580×ambient, with greater elevations observed in deeper (5-7cm) than surface (0-2cm) sediments. These enrichment levels were similar to eutrophic estuaries and were maintained for at least seven weeks. The high between-site variability could be partially explained by the sedimentary environment and macrofaunal community (42%), but only at the high application rate. We suggest future enrichment studies should be conducted in situ across large environmental gradients to incorporate real world complexity and increase generality of conclusions.

Same pattern, different mechanism: Locking onto the role of key species in seafloor ecosystem process

Seafloor biodiversity is a key mediator of ecosystem functioning, but its role is often excluded from global budgets or simplified to black boxes in models. New techniques allow quantification of the behavior of animals living below the sediment surface and assessment of the ecosystem consequences of complex interactions, yielding a better understanding of the role of seafloor animals in affecting key processes like primary productivity. Combining predictions based on natural history, behavior of key benthic species and environmental context allow assessment of differences in functioning and process, even when the measured ecosystem property in different systems is similar. Data from three sedimentary systems in New Zealand illustrate this. Analysis of the behaviors of the infaunal ecosystem engineers in each system revealed three very different mechanisms driving ecosystem function: density and excretion, sediment turnover and surface rugosity, and hydraulic activities and porewater bioadvection. Integrative metrics of ecosystem function in some cases differentiate among the systems (gross primary production) and in others do not (photosynthetic efficiency). Analyses based on behaviors and activities revealed important ecosystem functional differences and can dramatically improve our ability to model the impact of stressors on ecosystem and global processes.