8. Toward Biologically Diverse Seas

Biodiversity is crucial to a healthy marine environment

Marine organisms are at the foundation of functioning, productive ecosystems that support life on much of the planet, including human life. Understanding how and why ecosystems change is crucially important for developing effective and efficient management. To this end, OSPAR has greatly improved and expanded its assessments of marine biodiversity, by extending the focal areas to cover marine birds, marine mammals, fish, benthic habitats, pelagic habitats and marine food webs since the QSR 2010. Additionally, the assessments of biodiversity components for the QSR 2023 were based on a wider set of indicators and combined with status assessments and third-party assessments to provide better geographical coverage than previous assessments. This additional information from geographically wider and more integrated assessments of biodiversity helps to progress the NEAES 2030 objective of improving the information base and can guide the additional management actions needed to achieve OSPAR’s vision.

Collective trends point to declining biodiversity and continued habitat degradation across many parts of the OSPAR Maritime Area, even though the OSPAR Contracting Parties have taken measures to achieve clean, biodiverse, and productive seas. In particular, most marine birds, many threatened species of fish and some marine mammals are threatened by a combination of direct and indirect pressures, including from fisheries, hazardous substances, climate change and ocean acidification. Benthic habitats, especially in heavily used coastal areas, continue to be disturbed and degraded, while pelagic habitats are experiencing rapid transition through a combination of changes in nutrient availability and climate change effects. Since all these components are interlinked in the OSPAR Maritime Area (and beyond), impacts can be seen across entire marine food webs.

The QSR 2023 has provided more detailed evidence of how marine species are impacted by multiple direct and indirect pressures. Direct capture of marine organisms by fishing and hunting can drive loss of biodiversity, particularly if resource use is non-selective, poorly managed, or exceeds sustainable limits. Legacy pollutants, with their high chemical stability and re-release from sediment, are still contributing to the poor status of marine birds and marine mammals, even though concentrations of the most serious hazardous substances have decreased substantially. Increasing levels of human activities – both at sea and on land – are likely to have resulted in increased inputs of other substances. Aquaculture, shipping, recreational boating, and offshore industries continue to threaten marine species and food webs, particularly in some OSPAR Regions. Non-indigenous species (NIS) continue to enter and affect marine ecosystems in the OSPAR Maritime Area, including aquaculture escapees and organisms introduced via ships’ ballast water and biofouling. Noise pollution also impacts biodiversity. Pressures from human activities weaken marine ecosystems and reduce their resilience to climate change and ocean acidification, which are now driving major changes that imperil much of the North-East Atlantic’s marine biodiversity. Given these multiple pressures and cumulative impacts, additional limits on human activities - whether through expanded marine protected areas, improved fisheries measures, tourism management or the regulation of maritime industries - may be needed in order to keep our ocean healthy and productive.

Significant advances have been made by the OSPAR Contracting Parties to better understand and limit the negative impacts of human activity on the biologically rich North-East Atlantic. Through their fisheries authorities, OSPAR Contracting Parties have contributed to measures to protect vulnerable species and habitats. Environmental quality has improved in certain facets: concentrations of the most serious hazardous substances, such as PCBs, PAHs and organochlorides, have decreased substantially, pollution by radioactive substances has been prevented, pollution by the oil and gas industry has been reduced, marine litter is better tracked and significant steps have been taken to reduce it, and a gradual improvement in eutrophication has been attained in many OSPAR Regions. Cumulative pressures and ever-increasing climate change not only impact individual species and important functional groups of organisms, but also accelerate the spread of non-indigenous species, many of which can become invasive and further reduce biodiversity. This creates an ever-more pressing need to address the drivers of degradation and biodiversity loss and thereby increase the health and resilience of the many marine ecosystems in the OSPAR Maritime Area.

Marine Birds

Marine birds in the North-East Atlantic include seabirds that spend most of their lives at sea (petrels and shearwaters, gannets and cormorants, skuas, gulls, terns and auks) and waterbirds that mostly inhabit intertidal areas or inshore areas close by (waders, ducks, geese, swans, grebes and divers). Many of these species are in ‘not good’ status, with widespread declines in breeding productivity and population abundance observed in all the assessed OSPAR Regions. Most marine birds were not in good status in 2017 (at the time of the intermediate assessment) but additional deterioration in status has been observed during the assessment period for the QSR 2023.

The assessments of marine bird status are based on trends in 78 breeding and non-breeding (i.e. over-wintering or migrating) populations of species, grouped by feeding behaviour (surface, water column, benthic, wading or grazing feeders). Of the nine marine bird species identified by OSPAR as threatened and/or declining, five have been assessed for the QSR 2023 and all five are still declining; in addition, the Iberian breeding population of the common guillemot became extinct shortly after it was listed. Across the OSPAR Regions there are differences in the status of marine birds, with grazing feeders generally in better status than surface, water column, or benthic feeders. Integrated region-by-region assessments were based on two common indicator assessments: Marine Bird Abundance and Marine Bird Breeding Productivity .

Since most species of marine birds are predators, the availability of prey is a major factor in the maintenance of their populations. Climate change has affected prey availability for many species, especially surface feeders, and may be the single biggest pressure affecting marine birds in the North-East Atlantic. However, over-fishing of prey species has also played a part in some regions, as for example in the Greater North Sea, where fisheries are currently competing with seabirds for sandeels, whose populations are also declining in many parts of this Region. The direct effect of climate change on marine birds is evident from changes in the range and distribution of some species. There has been a north-eastward shift in the wintering range of many wader and wildfowl species (so-called “short-stopping”), leading to apparent declines in the Celtic Seas Region, which are partly offset by increases outside the OSPAR area in the Baltic Sea. The breeding ranges of some seabird species are predicted to shrink northwards, and this appears to be happening already (e.g. black-legged kittiwakes no longer breed in the Bay of Biscay).

Other anthropogenic pressures that lead to habitat loss, degradation, disturbance and direct mortality of marine birds have also driven the declines. Marine birds are accidentally caught and killed in fishing gear (commercial, recreational or artisanal) in the OSPAR Maritime Area. The population impacts of by-catch mortality on birds are largely unquantified, but there is some evidence to suggest that these could be significant in some areas and fisheries. Another cause of increased mortality in marine birds could be marine litter, through ingestion or entanglement, but the level of population impacts is currently unknown. Ingestion of plastic by fulmars is an indicator for OSPAR’s marine litter assessment, with data showing that ingestion is common.

Disturbance of breeding colonies also affects population status and reproductive success. Pressure from invasive predatory mammals causes loss of safe breeding habitat. The largest marine bird breeding colonies in all the OSPAR Regions occur on islands that are currently free of mammalian predators, but these are potentially at risk from future incursions, possibly facilitated by human activities such as tourism.

Greater attention to the rapidly changing marine environment has shed new light on multiple pressures affecting marine birds, including how localised pressures are exacerbated by climate change impacts on marine bird reproduction and survival. The key pressures on marine birds highlighted in QSR 2010 (and reiterated in the 2017 Intermediate Assessment) focused on declines in prey species, loss of and damage to habitats, and the introduction of non-indigenous species. The QSR 2023 shows that climate change is a key factor in marine bird decline, and that this is being exacerbated by habitat loss or degradation, disturbance in breeding colonies, and mortality due to by-catch. An emerging issue potentially affecting marine bird status – paradoxically driven by the need to combat climate change – is offshore renewable energy development that displaces foraging marine birds and may cause direct mortality from collisions. These impacts have yet to be adequately assessed but are likely to increase as offshore renewable energy production is expanded. OSPAR will produce an assessment of the cumulative impacts of offshore wind-farm development on bird populations in the near future.

A new threat has emerged in the form of Highly Pathogenic Avian Influenza (HPAI), which was assessed as relatively low-impact in the QSR 2023 thematic assessment before major impacts were noted in 2021-22 (after the assessment period ended). The 2021-2022 outbreak raised concerns by impacting seabird species which had been resilient to the virus in previous outbreaks.

Marine mammals

The marine mammals in the North-East Atlantic include pinnipeds (seals), small toothed cetaceans (dolphins/ porpoises, narwhals, belugas, killer whales), deep-diving toothed cetaceans (e.g. pilot whales and sperm whales) and baleen whales (e.g. humpback, minke, fin, sei, and blue whales). These species have been, and still are, subjected to significant pressures from both natural impacts and human activity, affecting many populations and species. Historical pressures, including whaling, reduced the populations of species such as the northern right whale to dangerously low levels, from which they have still not recovered. Many pressures still occur on a wide scale today, such as by-catch in fisheries, or are increasing, such as noise, hazardous substances and loss or degradation of habitat. Marine mammals tend to have wide distributional ranges and some species are very rare, making the monitoring of marine mammals challenging. This has led to our continuing low level of understanding about the distribution and population size of many marine mammal species, despite ongoing monitoring and research efforts.

Where qualitative and quantitative assessments of marine mammals have been done for the QSR 2023, the data suggest that many species and populations are not in good status. OSPAR has included four marine mammal species in the list of threatened and/or declining species: the bowhead whale, the northern right whale, the blue whale and the harbour porpoise. All four species in the assessed regions of the OSPAR Maritime Area are considered to be in poor status, and the northern right whale is considered to have been eliminated from the North-East Atlantic.

For other species of cetaceans and seals that were assessed ( Marine Mammals Thematic Assessment State chapter ), limited improvement has been observed as compared with previous assessments, though for many species it is simply not known (with confidence) how well they are faring. Integrated status assessments of two species groups, seals and small toothed cetaceans, were undertaken in the Greater North Sea, Celtic Seas and Bay of Biscay and Iberian Coast Regions. Small toothed cetaceans are not in good status in all regions where they were assessed. This is largely due to mortality from incidental by-catch within these Regions, but also to declining populations of coastal bottlenose dolphin and harbour porpoise in the Bay of Biscay and Iberian Coast Region, and harbour porpoise in the Celtic Seas Region. The status of baleen whales and deep-diving toothed cetaceans remains unknown owing to limited data.

The integrated assessment of seals in the Greater North Sea and Celtic Seas shows mixed results. Grey seals were found to be in good status in both Regions, while harbour seals were in not good status in the former but have unknown status in the latter, though for those sites where data were available, their abundance was increasing.

There are some indications of positive trends for certain marine mammal species in the Arctic. However, insufficient information on the Arctic Waters and Wider Atlantic Regions was available to permit a complete assessment.

The assessments presented in the QSR 2023 on the abundance and distribution of grey seals and harbour seals, on grey seal pup production, on marine mammal by-catch and on the distribution and abundance of cetaceans, identify multiple pressures on marine mammals. Exploitation has affected both groups; however, long-lived cetaceans encounter many more pressures across their lifetimes, including legacy pollutants, noise, and potential disturbance and ship strikes, across the large areas they inhabit. The assessments suggest that many pressures on marine mammals continue, and some are of increasing concern (for instance noise, plastic pollution, and hazardous substances). The QSR 2010 identified climate change, noise pollution, loss of habitat and prey, as well as by-catch, as the largest threats to marine mammals in the OSPAR area. It noted that cetaceans, especially baleen whales, were threatened, with the populations of some species depleted to near extinction and some not recovering, despite a moratorium on whaling. The QSR 2023 found that the main pressures on cetaceans identified in the QSR 2010 are continuing and in many cases increasing. Grey seal abundance was largely increasing across the regions assessed and grey seal pup production was rising in all assessment units where data were available, except in Icelandic waters, where production is decreasing. These reported increases may be attributed to the fact that grey seals are still under recovery from massive declines in the past due to hunting and pollution and are recolonizing previously utilised habitats. Harbour seals have faced historical pressures similar to those of grey seals; however, they have also been impacted by intermittent outbreaks of Phocine Distemper Virus (PDV) in some assessed regions.

A pilot assessment for the presence of persistent organic pollutants (polychlorinated biphenyls or PCBs) in 30 species of marine mammals at the OSPAR scale found them present in marine mammals living in all five OSPAR Regions. Toothed cetaceans and some subpopulations of seals present moderate to high ranges of PCB concentration, often surpassing the estimated toxicity thresholds for the onset of reproductive incapacity, while the ranges of PCB concentrations in baleen whales were below the estimated toxicity thresholds. This analysis was not included in the integrated assessment of marine mammal status but hints at higher PCB toxicity risk in small toothed cetaceans in the Greater North Sea, Celtic Seas and Bay of Biscay and Iberian Coast Regions, following the trends evidenced by the common indicators.

Marine mammals are very mobile and utilise a wide array of pelagic habitats. This means that MPAs will be insufficient as a protection measure unless they form ecologically coherent networks that can protect these mobile species at multiple (critical) life stages. OSPAR is moving in this direction, and mapping of migration and distribution patterns has revealed several key marine mammal hot spots - areas with high species richness and important for common resting, nursing, moulting and foraging areas - ( Marine Mammals Thematic Assessment ) that could be the focus of additional protection measures.

Fish

Marine fish are a key component of all North-East Atlantic marine ecosystems and contribute to most of the ecosystem services that our ocean provides. Assessing the status of fish in a comprehensive way requires monitoring of indicator species as well as making use of the vast amount of data collected by fisheries management organisations for those species that are commercially fished. OSPAR has for the first time incorporated information on commercial fish stocks into the integrated assessment on marine fish, to provide its most comprehensive fish status assessment to date.

OSPAR has directly or indirectly assessed the population status of 316 marine fish species in three OSPAR Regions: Greater North Sea, Celtic Seas, and Bay of Biscay and Iberian Coast. The Arctic Waters and Wider Atlantic Regions were not assessed. Fish species were classified as belonging to one of four groups: coastal, demersal, pelagic or deep-sea. The overall status of fish in the OSPAR Maritime Area was assessed by integrating the OSPAR common indicator for the recovery of sensitive fish species with third-party assessments of commercial fish stocks produced by the International Council for the Exploration of the Sea (ICES) and the International Commission for the Conservation of Atlantic Tunas (ICCAT). An integrated assessment of species/stock status found that marine fishes were not in good environmental status. This finding was common to coastal, demersal and pelagic fishes for all three assessed regions (Greater North Sea, Celtic Seas, Bay of Biscay and Iberian Coast) and for deep-sea fish across the OSPAR Maritime Area.

OSPAR has identified 22 fish species as under threat and/or in decline in the North-East Atlantic and has taken national and collective actions to protect and conserve most of these species. The ‘Recovery in the populations of sensitive fish species common indicator’ presents species-level outcomes for different fish populations in the OSPAR Maritime Area, in addition to the fish communities that are assessed in food web common indicator assessments. This species-level indicator addresses the extent of recovery among populations of fish deemed sensitive to additional mortality from fishing.

Fish species with life history traits such as large ultimate body size, slow growth rate, and late age at maturity are particularly sensitive to additional sources of mortality, including through by-catch. Populations of such species are known to have declined markedly in abundance throughout the twentieth century, a period of marked expansion in fishing activity across the area assessed. The findings are that 56 (49%) out of 114 assessed regional populations achieved their long-term threshold for “Recovering” and 93 (82%) met the secondary threshold of “No Further Decline”. One fifth of assessed populations (18%) failed to achieve either threshold, and half of those showed signs of recent decline (Recovery Sensitive Fish). Therefore, excepting some species of skates and rays, most of the OSPAR-listed fish species assessed across all their populations that could support assessment are currently deemed as not having achieved the threshold requiring that sensitive species should be recovering over the long term and that further decline of the species has been halted.

The dominant pressure on fish populations in the North-East Atlantic is extraction of biomass through fishing activities. Commercially fished species are generally managed to allow for sustained harvesting, with measures keeping extraction/harvest to levels at or below maximum sustainable yield (MSY) thresholds. Fisheries management regulations have resulted in many successes in the harvesting of fish stocks at levels considered sustainable for those stocks, and in rebuilding stocks to healthy levels. For example, a separate assessment for the Norwegian waters in the Arctic Waters Region showed mixed results, with commercial stocks in both good and not good status.

It is noteworthy that overall good environmental status has not been achieved for populations of fish in most of the OSPAR Maritime Area. Though most commercially fished stocks in the OSPAR Regions are considered sustainably managed, further studies are needed to better understand the impacts on wider biodiversity as well as on the target species. Marine fish are affected not only by fishing-related mortality, but also by habitat disturbance caused by certain fisheries. Fishing can reduce the biomass of fish populations to levels that negatively affect other species by decreasing prey availability or by causing food web imbalances, while also potentially affecting the age structure of populations of targeted species by reducing the proportion of older, larger and more fecund individuals. Among the fishing methods utilised, some have the potential to kill or remove non-target species through incidental catch (by-catch). Such by-catch can negatively impact fish species of conservation concern such as sharks, skates, and rays as well as other species such as marine mammals and marine birds. Furthermore, fishing methods contacting the seabed (bottom-trawl and dredge fisheries) can cause physical disturbance resulting in reduction of prey, resuspension of sediment and habitat loss. This can have a severe impact on OSPAR-listed threatened and/or declining habitats such as Lophelia pertusa reefs and maerl beds.

Marine Turtles

Two species of marine turtle (the loggerhead turtle and the leatherback turtle) regularly occur in the OSPAR Maritime Area. Though partial assessments have been completed for both, status determinations are not possible for either, given that limited data suggest both positive and negative trends. Loggerhead turtles (primarily juveniles) occur all year round in the wider Atlantic and the Bay of Biscay. In the latter, a peak of observations occurs in the winter, due in part to the stranding of cold-stunned individuals. The sightings of juvenile loggerheads in the wider Atlantic, especially around the Azores islands, have increased slightly, corresponding to the population increases evidenced at nesting beaches in Florida and elsewhere in North America.

Leatherback turtles forage throughout the OSPAR Maritime Area, with the Bay of Biscay and Iberian Peninsula identified as “high-use” areas and a seasonal pattern of occurrence showing a peak between August and October. In the Celtic Seas and Bay of Biscay and Iberian Coast Regions, peak occurrence appears to coincide with that of large and diverse aggregations of jellyfish documented in the Bay of Biscay and off south-west England and Wales. Observations of leatherbacks and the data on strandings point to a slight decline in the incidence of these turtles in the OSPAR Maritime Area. This corresponds to a general decline in the North Atlantic population, which is a relatively new development; the species is considered to be endangered throughout its global range.

The threats to both species include incidental capture in fisheries, contaminants, nesting habitat loss and degradation (including that due to climate change-related impacts), entanglement in/ ingestion of marine litter, and ship strikes. The last assessment of loggerheads in 2015 determined that by-catch in longline fisheries was likely to be the greatest pressure in the OSPAR Maritime Area. However, longline fishing has decreased in the region during the current assessment period, and the recent ban on wire leaders may also be mitigating this threat in the region. Fisheries interactions are the main driver of mortality in leatherbacks as well, resulting from entanglement in fishing pot lines, by-catch in pelagic longline, and incidental capture in net fisheries. Both species also suffer mortality and morbidity from ingesting plastic; there is such a high incidence of litter ingestion by sea turtles in the Bay of Biscay, in the Azores and in Macaronesia as a whole that the number of stranded loggerhead turtles having ingested plastic is used as an indicator for the general distribution of marine litter in the southern portion of the OSPAR Maritime Area. This new common indicator provides the baseline for further monitoring and evaluation of trends, and though thresholds have not yet been determined, the data suggest that the distribution of marine litter in some parts of the OSPAR Maritime Area is increasing.

Recommendations for the future regarding marine turtles in the OSPAR Maritime Area include conducting assessments every six years, enhancing ‘turtle codes’ to guide the reporting of turtles and the care of stranded individuals, developing common indicators for sea turtle by-catch, and expanding fishery observer programmes as well as multi-taxa at-sea surveys.

Benthic Habitats

Benthic habitats are essential for marine life, because marine species rely directly or indirectly on the seafloor in order to feed, hide, rest or reproduce. Benthic habitats are characterised by animal and plant communities with no or slow mobility when compared with fish or marine mammals. In the OSPAR Maritime Area, these habitats range from hard substrate to soft mud or sand and include biogenic habitats such as coral and shellfish reefs. These habitats have been and continue to be under threat, primarily in certain shelf areas. These threats come from a variety of pressures including disturbance caused by abrasion (bottom trawling), the temporary or permanent removal of substrate (during sand extraction, dredging and the erection of man-made structures), biological pressures from species harvesting or the spread of non-indigenous species, chemical pressures such as pollution by nutrients or contaminants, and the combined background pressures of climate change and ocean acidification.

The OSPAR quality status assessments of benthic habitats, both broad habitat types and OSPAR-listed benthic habitats, were undertaken using a variety of assessment methods and scales, developed over many years. Those areas with sufficient data were assessed using common indicators and candidate indicators, for agreed pilot studies.

For the Greater North Sea, Celtic Seas and Bay of Biscay and Iberian Coast Regions, quantitative approaches have been applied, using regional common indicators. The current assessment reports several continuous and wide-scale pressures from human activities affecting most benthic habitat types at various intensities in those areas where OSPAR common indicators have been applied. The indicator results show that benthic habitats continue to be impacted by bottom-trawling fisheries and other seafloor disturbances, and by eutrophication in several coastal habitats. Due to the complexity of benthic ecosystems, a method for aggregating indicator outputs could not be determined in time to enable an overall assessment of quality status; this will be further explored in future work. No common OSPAR indicators were agreed for the Arctic Waters Region, and therefore the area was assessed using third party assessments, notably from the Arctic Council ( State of the Arctic Marine Biodiversity Report ) and ICES, combined with expert judgement. The trends detected are mainly related to climatic impacts, together with some evidence from other pressures ( Benthic Habitats Thematic Assessment State Chapter ).

The QSR 2023 reports that 17 out of the 18 benthic habitats on the OSPAR List of Threatened and/or Declining Species and Habitats are in poor status in the OSPAR Maritime Area; however, some habitats are in good or unknown status in some of the Regions ( Benthic Habitats Thematic Assessment ).

Oceanic ridges were assessed to be in good condition, which is an improvement since the QSR 2010. In the Arctic Waters Region, deep-sea sponges and Lophelia pertusa reefs have not been comprehensively mapped in all areas, but in places where long term monitoring is ongoing, deep-sea sponge beds have shown evidence of decline, whereas Lophelia pertusa reefs within MPAs are shown to be in good status. The carbonate mounds in the Wider Atlantic Region are the largest known globally, and since the QSR 2010 new mounds have been discovered on the Iberian margin of the Galicia Bank. A wider range of threatened and/or listed habitat types have been found to be associated with carbonate mounds than was recognised during the last assessment. Where monitored, deep-sea sponge aggregations, Lophelia pertusa reefs and seamount communities show evidence of decline.

Many of the benthic habitats in the OSPAR Maritime Area are highly sensitive and therefore vulnerable. The background pressures from climate change and ocean acidification, together with the increasing trend in aquaculture and renewable energy production at sea, mean that it is urgent and vitally important to protect sensitive habitats. Furthermore, it will be necessary to decrease the current level of pressures on benthic habitats that have been compromised in their quality, function and ability to provide associated ecosystem services.

Pelagic Habitats

Pelagic habitats and the plankton they harbour are essential components of the rich and diverse North-East Atlantic. Plankton, both phytoplankton (e.g. single-celled microscopic algae such as diatoms) and zooplankton (e.g. copepods and the larvae of fish and benthic invertebrates), occupy the lower tiers of the food web and are the main source of marine production. Phytoplankton have very short life cycles, making plankton particularly sensitive to environmental change. Changes in plankton communities can also affect higher food web levels such as shellfish, fish, and seabirds, which they support either directly or indirectly.

OSPAR assesses pelagic habitats across four types of open-water habitats: variable salinity habitats, coastal habitats, shelf habitats, and oceanic habitats beyond the continental shelf. Their distribution is shown in Figure 8.4.

Pelagic habitats in the OSPAR Maritime Area have experienced widespread changes over the past 60 years, with indicator assessments revealing a general pattern of decreasing phytoplankton and zooplankton abundance and/or biomass across the Greater North Sea, Celtic Seas, and Bay of Biscay and Iberian Coast Regions. Long-term trends have largely continued into the current assessment period and are expected to continue into the future, eventually impacting higher food web levels. Due to widespread changes linked to pressures generated by human activities, pelagic habitats in these three Regions are judged to be in “not good” status (Figure 8.4).

The growing global population has generated increasing demand for food production, waste disposal, coastal development and energy systems, all of which contribute to human-induced climate change. Climate change (both from natural variability and human-induced) is probably the greatest pressure currently impacting plankton communities across the OSPAR Maritime Area as a whole. Sea surface warming has been statistically linked to changes in plankton communities, ( Pelagic Habitats Thematic Assessment ) which implies that a causal relationship might exist. This suggests that as climate change continues, so will changes in plankton communities. These activities also influence the supply of nutrients entering coastal environments, which can generate eutrophication and impact the productivity of pelagic habitats.

Global efforts to slow climate change are the best mechanism for countering widespread changes in plankton communities, but OSPAR can do much to remove other pressures on pelagic habitats. Regionally targeted management measures (e.g. controlling inputs of nutrients and organic matter) in coastal areas may affect pelagic habitats at the shelf scale. While these mitigation efforts are likely to only generate noticeable impact in coastal areas, they may also have some effect in areas where plankton communities are affected by the cumulative impacts of multiple pressures (i.e. both warming and eutrophication).

Plankton, both phytoplankton that include diatoms (left) and zooplankton (e.g. the larvae of fish and benthic invertebrates), occupy the lower tiers of the food web and are the main source of marine production © Shutterstock

Marine Food Webs

Food webs account for all biota that are linked by energy transfer in an ecosystem. Intact and functioning food webs are essential for healthy, productive, diverse and valuable marine systems.

Environmental and anthropogenic pressures alter the balance between organisms. Any human activity that has an impact on biota in our marine ecosystems is also likely to have an impact on food webs, and this impact can be local or far away from the site of the pressure. At the base of marine food webs, plankton provide the foundation for most sea life. The flux in primary production in the OSPAR Maritime Area is mainly driven by reduced nutrient availability and increasing sea surface temperatures, and this then affects other plankton functional groups. Higher up in the food web, declines in the fish communities are mainly linked to fishing pressures, though food web alterations can also play a role in fish populations. Apex predators at the top of food webs depend on productivity and balance in the food web below, but also exert some top-down control on productivity. Intact marine food webs support the diversity of life in our seas and ocean and, importantly, they also support many of the ecosystem services upon which humans depend.

Knowledge of the structure and functioning of food webs is therefore essential, not only for understanding marine ecosystem functioning and how it may be conserved or restored but also to enable sustainable use of our seas in the future. For the QSR 2023, OSPAR has assessed a set of food web indicators showing that the abundance, distribution, and productivity of key groups representing different trophic levels are in flux in many of the assessed areas. The first and second layers of food webs were assessed by analysing production of phytoplankton, changes in zooplankton and ratios between plankton lifeforms. Fish communities were also assessed in terms of size structure, species composition and the biomass of feeding guilds of fish, linking the lower trophic levels to predators. The effects of fishing pressures on the structure of the demersal fish community were analysed by assessing the status of different trophic levels including mesopredators and top predators. Finally, in a pilot study, multiple food web components and their interactions were integrated through a modelling approach.

The overall results for food webs suggest that good status has not been achieved. Food web structure and functioning varies between OSPAR Regions (Figure 8.5). In the Arctic Waters Region, where the common indicators were not available, the assessment used evidence taken from other sources (Arctic Council / AMAP / ICES) and indicates that climate change is currently the strongest driver of change in the marine food web. In Greater North Sea, the indicators for demersal fish communities have not achieved good status, signifying that this component of the food web is under pressure. The plankton community indicators show a decreasing trend for primary production and a mixed picture for various plankton lifeforms, with some showing increasing trends and other decreasing. In the Celtic Seas Region, the indicators for demersal fish community structure have not achieved good status, indicating a long-term decline in the Region. The plankton community indicators show a decreasing trend for primary production and a decreasing trend for virtually all plankton lifeforms. In the Bay of Biscay and Iberian Coast Region, the indicators for demersal fish community structure demonstrate no clear patterns; however, the fish feeding guild indicator shows that the biomass of fish guilds was generally highest in the northern part of the Region. In the shelf areas of the Wider Atlantic Region, the indicators for demersal fish communities demonstrate no long-term changes; however, a pilot assessment of the multitrophic level indicator conducted in the Azores shows a decreasing trend for biomass at all tropic levels in this food web model, and the resilience of the ecosystem in this specific area may be decreasing over time.