Mass balanced models of the food web in three areas along a gradient of eutrophication symptoms in the south arm of the Mondego estuary (Portugal)

Trophic functioning of a small, anthropogenically disturbed, tropical estuary

A trophic model was constructed for the Poonthura Estuary, a small, anthropogenically impacted estuary along the south-western coast of India. An Ecopath with Ecosim based trophic modelling approach, based on observations made between 2016 and 2020, revealed that the Poonthura Estuary had a low total system throughput (3044.2 t km-2 year-1), low ascendancy (15%), high Finn's cycling index (17.9%), low primary production/total biomass (5.2 t km-2 year-1), high mean transfer efficiency (12.4%), and low eco-exergy (14,455.46 gm detritus equivalent m-2). These values indicated that the estuary is an immature, less organized, and unhealthy system. The evaluation of Ecological Network Analysis, and ecosystem health indices revealed that the ecological structure and functioning of the estuary are impaired to a large extent from multiple anthropogenic stressors. The Poonthura Estuary trophic model revealed the total primary production/respiration value as 0.46, indicative of the massive pollution that the system is subjected to, particularly from organic sources. Small benthic carnivores were the most important keystone groups recorded from the Poonthura Estuary, despite their low biomass. The comparison of ecological indices of Poonthura Estuary, with those recorded for other small estuaries from various geographical locations, suggested dissimilar trophic functioning and food web structures from estuaries with similar physical features. Our study is a pioneering step to reveal the ecosystem status and functioning of small, anthropogenically disturbed estuaries, besides offering theoretical and scientific basis for the management, supervision and restoration of the Poonthura Estuary as well as other small estuaries, around the world.

Biological processes in seagrass beds of coastal lagoons to maintain estuary-dependent marine fisheries

Following the planned FAO Ecosystem Restoration Programme for estuarine habitats to support estuarine fisheries and early life stages of estuary-dependent marine fish, direct relationships of total seagrass and eelgrass Zostera m. capricorni areas and biomass with fish harvest were derived for a range of slightly to highly urbanized coastal lagoons that are expected to support the larvae and juveniles of estuary-dependent marine fisheries. Fish harvest and seagrass area and biomass in the lagoons increased with moderate catchment total suspended sediment and total phosphorus loads due to lagoon flushing rates directing excess silt and nutrients out to sea via the lagoon entrances. Well managed, sewered catchment management works are shown that could assist estuary managers maintain seagrass for estuarine and offshore estuary-dependent fisheries by maintenance of seagrass and fishery ecological processes. Further research is suggested to investigate estuary-dependent post-juveniles leaving estuaries and lagoons migrating to nearshore, offshore and shelf marine fisheries.

Exploring multiple stressor effects with Ecopath, Ecosim, and Ecospace: Research designs, modeling techniques, and future directions

Understanding the cumulative effects of multiple stressors is a research priority in environmental science. Ecological models are a key component of tackling this challenge because they can simulate interactions between the components of an ecosystem. Here, we ask, how has the popular modeling platform Ecopath with Ecosim (EwE) been used to model human impacts related to climate change, land and sea use, pollution, and invasive species? We conducted a literature review encompassing 166 studies covering stressors other than fishing mostly in aquatic ecosystems. The most modeled stressors were physical climate change (60 studies), species introductions (22), habitat loss (21), and eutrophication (20), using a range of modeling techniques. Despite this comprehensive coverage, we identified four gaps that must be filled to harness the potential of EwE for studying multiple stressor effects. First, only 12% of studies investigated three or more stressors, with most studies focusing on single stressors. Furthermore, many studies modeled only one of many pathways through which each stressor is known to affect ecosystems. Second, various methods have been applied to define environmental response functions representing the effects of single stressors on species groups. These functions can have a large effect on the simulated ecological changes, but best practices for deriving them are yet to emerge. Third, human dimensions of environmental change - except for fisheries - were rarely considered. Fourth, only 3% of studies used statistical research designs that allow attribution of simulated ecosystem changes to stressors' direct effects and interactions, such as factorial (computational) experiments. None made full use of the statistical possibilities that arise when simulations can be repeated many times with controlled changes to the inputs. We argue that all four gaps are feasibly filled by integrating ecological modeling with advances in other subfields of environmental science and in computational statistics.

Influence of the river flow pulse on the maturity, resilience, and sustainability of tropical coastal ecosystems

Coastal marine ecosystems have structural and functional features usually connected by the seasonal transfer of nutrients and organisms. These environments can utilize inter-ecosystem subsidies to increase resilience and maturity and support human activities like fishing. However, the importance of the connection and the role of the seasonal pulse of energy flows to enhance maturity are still poorly understood and reported. Our objective in this paper is to assess the effect of seasonal hydrological pulses on two tropical coastal interconnected ecosystems. Thus, we made four Ecopath models for estuarine and neritic environments considering the dry and rainy seasons, with a similar sampling design that allowed them to be compared. Our results provide evidence for the occurrence of the pulsed ecosystems since both environments seem driven by the river flow. Estuary presents more and more substantial differences (measured by ecosystem attributes) in both seasons because it is directly affected by river floods than the neritic environment. The neritic is affected indirectly by the movement of species from the estuary and by a weaker river flow. In the dry season, the differences between ecosystems are lower because the dry season trend to homogenize cycling, maturity, homeostasis, and resilience. We found that the seasonal river flow (pulse) forces the variability of biomass, flows, and ecosystem features, and this variance creates the required stability for both ecosystems. Still, these environments benefit through the exchange of components that relieve the pressures of predation on specific groups and maintain the energy flow necessary for the functioning of their trophic webs. The pulse by the rainfall favors connectivity and equalizes the two systems, increasing the connectivity between them and the exchange of subsidies that strengthens the trophic structures, contributing to the increase in maturity. In these ecosystems, seasonal changes become a key factor for exchanging flows that will promote sustainability, the accumulation of more biomass (growth), and the optimization of reserve energy (development) in both systems. This efficient joint strategy of perpetuation is what promotes resistance and resilience to these ecosystems, which together can reach different states of equilibrium, translated into maturity to withstand new environmental changes.

Ecosystem health status and trophic modeling of an anthropogenically impacted small tropical estuary along India's west coast

The tropical estuaries are characterized with high biological production and also impacted by anthropogenic activities. Describing these estuaries in terms of ecological data and trophic dynamics to reveal the ecological impacts is gaining attention recently. In this study, the ecological structure is analyzed for a heavily impacted small macrotidal tropical estuary, Ulhas river estuary (URE), situated near Mumbai megacity in the western coast of India, to delineate the impact of anthropogenic stressors on the ecosystem functioning. The URE is being exploited for sand and fisheries resources, and also faces risks from anthropogenic activities. The ecological data of URE were compiled for 2017-18 together with the most relevant literature estimates to construct an ecosystem model. A trophic organization in 20 functional groups was identified for URE using Ecopath modeling approach. The functional groups identified in the food web ranged from detritus and primary producers (trophic level (TL) = 1) to large pelagics (TL = 4.14). Detritivory: herbivory ratio (1.35) indicated that the detritus chain is dominant over the primary producer's chain. The total system throughput (TST) was estimated as 16 736.2 t km^-2year^-1. The indices such as net system production (NSP = 1 398.781 t km^-2 year^-1), total primary production/total biomass (TPP/TB = 25.17), biomass/total system throughput (TB/TST = 0.01), recycling index (Finn's Cycling Index = 13.94%), system omnivory index (0.3), relative ascendency (25.6%), and system overhead (74.4%) classified URE as an immature system. The eco-exergy index (30748.54 gm detritus equivalent m^-2) showed that the ecosystem is a moderately stable and relatively less organized network. The estuarine fish community index (EFCI) yielded a value of 38 indicating the poor health status of the fish community in URE. The study delivers a comprehensive understanding of the ecosystem setting in URE and characterizes the prevailing condition. The ecological indicators analyzed here point towards a medium to a high level of impact in URE due to anthropogenic activities.

Biodiversity, functional redundancy and system stability: subtle connections

The relationship between biodiversity and functional redundancy has remained ambiguous for over a half-century, likely due to an inability to distinguish between positivist and apophatic (that which is missing) properties of ecosystems. Apophases are best addressed by mathematics that is predicated upon absence, such as information theory. More than 40 years ago, the conditional entropy of a flow network was proposed as a formulaic way to quantify trophic functional redundancy, an advance that has remained relatively unappreciated. When applied to a collection of 25 fully quantified trophic networks, this authoritative index correlates only poorly and transitively with conventional Hill numbers used to represent biodiversity. Despite such a weak connection, the underlying biomass distribution remains useful in conjunction with the qualitative diets of system components for providing a quick and satisfactory emulation of a system's functional redundancy. Furthermore, an information-theoretic cognate of the Wigner Semicircle Rule can be formulated using network conditional entropy to provide clues to the relative stability of any ecosystem under study. The necessity for a balance between positivist and apophatic attributes pertains to the functioning of a host of other living ensemble systems.

Modelling the marine eutrophication: A review

In the frame of a national, joint scientific appraisal, 45 scientific French-speaking experts have been mandated in 2015-2016 by the French ministries of Environment and Agriculture to perform a global review of scientific literature dealing with the eutrophication phenomenon, in freshwater as well as in marine waters. This paper summarizes the main results of this review restricted to a sub-domain, the modelling approach of the marine eutrophication. After recalling the different aims pursued, an overview is given on the historical time course of this modelling effort, its world distribution and the various tools used. Then, the main results obtained are examined, highlighting the specific strengths and weaknesses of the present models. Needs for future improvement are then listed.

Compliance of secondary production and eco-exergy as indicators of benthic macroinvertebrates assemblages' response to canopy cover conditions in Neotropical headwater streams

Riparian vegetation cover influences benthic assemblages structure and functioning in headwater streams, as it regulates light availability and autochthonous primary production in these ecosystems.Secondary production, diversity, and exergy-based indicators were applied in capturing how riparian cover influences the structure and functioning of benthic macroinvertebrate assemblages in tropical headwater streams. Four hypotheses were tested: (1) open canopy will determine the occurrence of higher diversity in benthic macroinvertebrate assemblages; (2) streams with open canopy will exhibit more complex benthic macroinvertebrate communities (in terms of information embedded in the organisms' biomass); (3) in streams with open canopy benthic macroinvertebrate assemblages will be more efficient in using the available resources to build structure, which will be reflected by higher eco-exergy values; (4) benthic assemblages in streams with open canopy will exhibit more secondary productivity. We selected eight non-impacted headwater streams, four shaded and four with open canopy, all located in the Neotropical savannah (Cerrado) of southeastern Brazil. Open canopy streams consistently exhibited significantly higher eco-exergy and instant secondary production values, exemplifying that these streams may support more complex and productive benthic macroinvertebrate assemblages. Nevertheless, diversity indices and specific eco-exergy were not significantly different in shaded and open canopy streams. Since all the studied streams were selected for being considered as non-impacted, this suggests that the potential represented by more available food resources was not used to build a more complex dissipative structure. These results illustrate the role and importance of the canopy cover characteristics on the structure and functioning of benthic macroinvertebrate assemblages in tropical headwater streams, while autochthonous production appears to play a crucial role as food source for benthic macroinvertebrates. This study also highlights the possible application of thermodynamic based indicators as tools to guide environmental managers in developing and implementing policies in the neotropical savannah.

Trophic linkage of a temperate intertidal macrobenthic food web under opportunistic macroalgal blooms: A stable isotope approach

The effects of blooms of opportunistic green macroalgae, Ulva prolifera, on the trophic structure of the macrobenthic food web in a temperate intertidal zone on the western coast of Korea were evaluated using carbon and nitrogen stable isotopes. Biomasses of Ulva and microphytobenthos (MPB) increased significantly at the macroalgae-bloom and the non-bloom sites, respectively, from March to September 2011. The δ(13)C values of most the consumers were arrayed between those of MPB and Ulva at both sites, and differed according to feeding strategies at the macroalgae-bloom site. Seasonally increasing magnitudes in δ(13)C and δ(15)N values of consumers were much steeper at the macroalgae-bloom site than at the non-bloom site. Our findings provide evidence that blooming green macroalgae play a significant role as a basal resource supporting the intertidal macrobenthic food web and their significance varies with feeding strategies of consumers as well as the resource availability.

A novel characterization of amalgamated networks in natural systems

Densely-connected networks are prominent among natural systems, exhibiting structural characteristics often optimized for biological function. To reveal such features in highly-connected networks, we introduce a new network characterization determined by a decomposition of network-connectivity into low-rank and sparse components. Based on these components, we discover a new class of networks we define as amalgamated networks, which exhibit large functional groups and dense connectivity. Analyzing recent experimental findings on cerebral cortex, food-web, and gene regulatory networks, we establish the unique importance of amalgamated networks in fostering biologically advantageous properties, including rapid communication among nodes, structural stability under attacks, and separation of network activity into distinct functional modules. We further observe that our network characterization is scalable with network size and connectivity, thereby identifying robust features significant to diverse physical systems, which are typically undetectable by conventional characterizations of connectivity. We expect that studying the amalgamation properties of biological networks may offer new insights into understanding their structure-function relationships.

The contribution of benthic macrofauna to the nutrient filter in coastal lagoons

Human activities in coastal areas have increased the occurrence of eutrophication events, especially in vulnerable ecosystems such as coastal lagoons. Although we have a general knowledge of the consequences of eutrophication in these ecosystems, some efforts need to be made to understand biotic feedbacks that could modify the response of the environment to nutrient enrichment. The plant-mediated 'coastal filter' is one of the main factors that determine lagoonal efficiency in processing excess nutrients. In this context, the present paper examined the relative contribution of benthic macrofauna to the 'coastal filter' of a Mediterranean lagoon. The analysis of macrofaunal assemblages in the Mar Menor lagoon led to a clear differentiation between shallow areas of net nutrient recycling and exportation and deeper areas of net retention. These differences enhance nutrient removal from the water column, thus increasing the ecosystem's resistance to eutrophication.

Food-web structure of seagrass communities across different spatial scales and human impacts

Seagrass beds provide important habitat for a wide range of marine species but are threatened by multiple human impacts in coastal waters. Although seagrass communities have been well-studied in the field, a quantification of their food-web structure and functioning, and how these change across space and human impacts has been lacking. Motivated by extensive field surveys and literature information, we analyzed the structural features of food webs associated with Zostera marina across 16 study sites in 3 provinces in Atlantic Canada. Our goals were to (i) quantify differences in food-web structure across local and regional scales and human impacts, (ii) assess the robustness of seagrass webs to simulated species loss, and (iii) compare food-web structure in temperate Atlantic seagrass beds with those of other aquatic ecosystems. We constructed individual food webs for each study site and cumulative webs for each province and the entire region based on presence/absence of species, and calculated 16 structural properties for each web. Our results indicate that food-web structure was similar among low impact sites across regions. With increasing human impacts associated with eutrophication, however, food-web structure show evidence of degradation as indicated by fewer trophic groups, lower maximum trophic level of the highest top predator, fewer trophic links connecting top to basal species, higher fractions of herbivores and intermediate consumers, and higher number of prey per species. These structural changes translate into functional changes with impacted sites being less robust to simulated species loss. Temperate Atlantic seagrass webs are similar to a tropical seagrass web, yet differed from other aquatic webs, suggesting consistent food-web characteristics across seagrass ecosystems in different regions. Our study illustrates that food-web structure and functioning of seagrass habitats change with human impacts and that the spatial scale of food-web analysis is critical for determining results.

The robustness of ecological indicators to detect long-term changes in the macrobenthos of estuarine systems

Accurate and reliable benthic quality indicators are in great demand following the recent developments and the strict time schedule for implementing the European Water Framework Directive. The Mondego estuary has experienced a progressive deterioration during the 1990s, followed by a partial ecological recovery due to restoration measures in 1997/1998. We have used the estuary as a model system to test the performance and robustness of a set of ecological indicators in highlighting the changes in the ecological state of intertidal areas. Over a period of 17 years (1985-2002), we calculated Margalef, Shannon-Wiener, Berger-Parker, Taxonomic Distinctness measures, AZTI's Marine Biotic Index, Infaunal Trophic Index, and Eco-Exergy based indices and tested differences across periods characterised by different anthropogenic disturbance. We combined temporal data within three periods: before, during and after disturbance, based on progressive information on the changes in the extended type of anthropogenic disturbance. Indices were then compared with biological and abiotic descriptors (macroalgae, macrophytes, benthic macrofauna, nutrients concentration, sediment grain size and total organic carbon). We found great disparity in the indicators ability to capture temporal changes, showing distinct performances at each site. At the Zostera noltii site, only Margalef, Total Taxonomic Distinctness and the thermodynamically based indices captured temporal changes, despite giving higher values during the disturbance period. At the bare sediment site, Taxonomic Distinctness, ITI, Shannon-Wiener, Berger-Parker, AMBI and the TBI were able to distinguish between periods, in agreement with the differences observed analysing the macrobenthic assemblages. Furthermore, Taxonomic Distinctness was not robust enough to detect any temporal or spatial change. We thus suggest further research to understand the behaviour of ecological indicators, in view of their crucial importance for the management and protection of marine coastal areas.

Opportunistic macroalgae metrics for transitional waters. Testing tools to assess ecological quality status in Portugal

Macroalgae communities constitute one of the ecological quality elements for the evaluation of the ecological quality status (EQS) of coastal and transitional waters, required to implement the WFD. While these algae are natural components of estuarine systems and play important roles in several estuarine processes, macroalgal blooms are of ecological concern because they can reduce the habitat quality. Several works are being carried out to set standard methods for monitoring macroalgae blooms, in order to develop tools to derive EQS based upon this biological quality element. The aim of this paper is to apply the methodology described by Scanlan et al. [Scanlan, C.M., Foden, J., Wells, E., Best, M.A., 2007. The monitoring of opportunistic macroalgal blooms for the water framework directive. Marine Pollution Bulletin 55, 162-171] to a series of data assembled in the south arm of the Mondego estuary (Atlantic coast of Portugal) considering two different ecological situations. Additionally, an alternative assessment method intended to be used when no biomass data are available was also tested. In general, both options captured the inter-annual variations in accordance with the system evolution. Option 2, less expensive and time-consuming, allowed an EQS evaluation with accurate results when biomass data were not available. The results suggest that sampling should be carried out from April to June.