Pelagic habitats are open-water environments occupied by floating and suspended organisms, or simply plankton, which occupy the lower tiers of the food web and are the main source of marine production. Short generational times, small size and, for phytoplankton, direct dependence on dissolved nutrients, make 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.
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. These activities also influence the supply of nutrients entering coastal environments, which can generate eutrophication and impact the productivity of pelagic habitats.
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. 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, the Greater North Sea, Celtic Seas, and Bay of Biscay and Iberian Coast had not good status, under the current definition and categorisation of quality status.
Global efforts to slow climate change are probably the best mechanism to counter widespread changes in plankton communities, although effective measures for reducing or preventing climate change mostly lie outside the remit of OSPAR. 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 only likely to 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).
Q1. Identify the problems? Are they the same in all OSPAR regions?
Pelagic habitats are open-water environments occupied by floating and suspended organisms, or simply plankton, which occupy the lower tiers of the food web and are the main source of marine production. These organisms include phytoplankton (e.g., green algae and diatoms) and zooplankton (e.g., copepods and the larvae of fish and benthic invertebrates), but not larger organisms such as pelagic fish.
Compared with more complex organisms, plankton typically have fewer mechanisms to buffer themselves against environmental change, and less mobility to evade adverse conditions. Moreover, they tend to be fast-growing due to short generation times. For these reasons, plankton communities are considered excellent sentinels of environmental change, most notably changes in climate variables and nutrient availability.
The most important human activities affecting pelagic habitats are therefore those that affect the nutrient status of marine waters (such as agriculture, aquaculture, and waste-water treatment) and those leading to climate change (any process involving burning of fossil fuels, agriculture, or deforestation) or that attempt to reduce climate change (including investment in renewable energy). Pelagic habitats were not specifically assessed in the Quality Status Report 2010 (QSR 2010), although climate change was flagged as a threat to marine habitats in general.
Plankton communities that form pelagic habitats are also not currently considered within Marine Protected Area (MPA) status assessment or the Environmental Impact Assessment (EIA) guidance. There is recent evidence that offshore wind infrastructure can impact the productivity of pelagic habitats; however, this is not currently a consideration for new offshore wind projects.
The Intermediate Assessment 2017 introduced for the first time a set of pelagic habitat indicators, which revealed trends in pelagic community composition and productivity and were primarily linked to climate change. In the 2017 assessment there were no threshold values or conditions to define the ecological status of pelagic habitats, and that situation has not changed. For the current assessment, detected trend linked to human activity (not good) status was assigned when the following conditions were met: change in an indicator was detected, the indicator demonstrated high correlation with pressures linked to human activities (e.g. warming, nutrients), and when there was high spatial and temporal confidence among the evaluated time-series.
It is expected that pressures from climate change (and also from ocean acidification) will intensify in the near future and persist for the longer term. Changes in the physicochemical conditions of the water column as a result of these pressures are likely to disturb plankton communities, impacting the relative abundance of plankton functional groups (e.g., diatoms vs dinoflagellates, holoplankton vs meroplankton) as well as the occurrence of planktonic species (groups). This may in turn have an impact on the ecosystem's carrying capacity and create knock-on effects for organisms higher up the marine food web.
Q2. What has been done?
There are no OSPAR measures which directly address problems in pelagic habitats. Climate change is the most important factor currently affecting plankton communities in the OSPAR Maritime Area. Effective measures for reducing or preventing climate change lie outside the remit of OSPAR, except for North-East Atlantic Environment Strategy 2030 strategic objective 12, which addresses the safeguarding of natural carbon stores.
Nutrient input is, however, an area for which OSPAR measures have been put in place. Starting in 1988, OSPAR has implemented programmes to reduce nutrient inputs (see: Eutrophication Thematic Assessment for details) and this area of work has been taken up in EU legislation. The ambition to manage nutrient pressure remains high. NEAES 2030 includes the strategic objective to “tackle eutrophication through limiting inputs of nutrients and organic matter”.
Other relevant human activities are regulated through the competent organisations that manage shipping and fisheries.
Q3. Did it work?
No responses specifically address pelagic habitat quality, but some measures aimed at eutrophication impact pelagic habitats. With the increasing success of OSPAR’s ability to assess this ecosystem component comes the need to further develop scientific understanding of the ecological consequences which can result from variation in the physical drivers of plankton production.
Since their introduction in the late 1980s, OSPAR’s long-term programmes and measures to limit nutrient inputs have been those with the greatest impact in improving pelagic habitat quality. While these nutrient reductions have had the positive effect of reducing the prevalence of eutrophication, there has been little change in the number of assessment units and surface area with elevated chlorophyll concentrations since the Intermediate Assessment 2017.
The pelagic habitats indicator assessments also concluded that it is likely that some measures put in place to control nutrient inputs in the Greater North Sea and Celtic Seas may have the consequence of limiting ecosystem productivity in shelf areas.
Q4. How does this field affect the overall quality status?
Pelagic habitats, and the planktonic organisms they support, provide most of the energy responsible for sustaining marine food webs. Therefore, large-scale shifts in plankton have the potential to impact entire ecosystems. Trends in plankton detected with pelagic habitat indicators can reveal important changes in the quality status of marine ecosystems. The assessments presented in the current edition of the QSR report on changes in the abundance of important plankton functional groups, the abundance of copepods and biomass of phytoplankton, and finally changes in phytoplankton and zooplankton community composition through species turnover and changes in dominance.
The Indicator Assessments revealed that pelagic habitats in the OSPAR Maritime Area have experienced widespread changes over the past 60 years. There was 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. Many of the observed changes were statistically linked to climate change (both from natural variability and human-induced), probably the greatest pressure currently impacting pelagic habitats. Additionally, measures put in place to control eutrophication in the Greater North Sea and Celtic Seas are intended to reduce productivity.
Despite the recent advancements in assessment methods, it is important to note that there were few suitable datasets available for assessing changes in gelatinous zooplankton, and in zooplankton in variable salinity habitats, due in both cases to insufficient monitoring.
While pelagic habitats were not formally assessed in 2010 and were not assessed with the same level of sophistication in 2017, long-term trends have for the most part continued into the current assessment period.
Due to widespread changes linked to pressures generated by human activities, the Greater North Sea, Celtic Seas, and Bay of Biscay and Iberian Coast were assigned not good environmental status, under the current definition and categorisation of quality status.
Q5. What do we do next?
In NEAES strategic objective 1, OSPAR has committed to “tackle eutrophication, through limiting inputs of nutrients and organic matter to levels that do not give rise to adverse effects on the marine environment”. This presents an opportunity to address a key pressure affecting the status of pelagic habitats. Future assessments must consider how to include plankton community indicators directly as part of the eutrophication assessment. The assessment needs to build the evidence of how nitrate and phosphate ratios lead to changes in phytoplankton communities (which negatively impact the efficiency of the ecosystem services those communities provide).
Furthermore, there may be opportunities to explore how to consider plankton communities that form pelagic habitats within the MPA status assessment, and to consider including plankton community dynamics in the EIA guidance. Given the current scaling-up of offshore renewable energy infrastructure, it is also important to consider the effects this will have on plankton dynamics.
In the future, OSPAR’s ongoing work to improve understanding of linkages across trophic levels (top-down and bottom-up) should incorporate the concept of trophic cascades, with particular focus on links to plankton health, food web function and derived ecosystem services.
Finally, future assessments should aim to quantify the effects of plankton change on ecosystem services and the effects of pressures on plankton as natural capital. Both should be integrated into policy development (OSPAR Science Agenda / Knowledge gap).
Pelagic Habitats Assessments
Lead authors: Abigail McQuatters-Gollop, Matthew Holland, Arnaud Louchart, Luis Felipe Artigas
Supporting authors: Shannon White, Federico Cornacchia, Julien Favier, Eric Goberville, Emily Corcoran, Birgit Heyden, Janos Hennicke, Daniel Wood, Angus Atkinson, Eileen Bresnan, Isabelle Rombouts, Margarita Machairopoulou, Rita Pires, Paulo Oliveira, Marie Johansen, Gavin Tilstone, Jos Schilder, Rafael González-Quirós, Jeanette Göebel, Rowena Stern, Lena Avellan, Michelle Devlin, Eva Verkevisser, Gro van der Meeren, Dorothee Vincent
Supported by: Biodiversity Committee (BDC), Intersessional Correspondence Group on the Quality Status Report (ICG-QSR), Intersessional Correspondance Group on Ecosystem Assessment Outlook (ICG-EcoC), Intersessional Correspondence Group on Economic and Social Analysis (ICG-ESA), Climate Change Expert Group (CCEG), and OSPAR Commission Secretariat
This work was co-funded by the European Maritime and Fisheries Fund through the project: “North-east Atlantic project on biodiversity and eutrophication assessment integration and creation of effective measures (NEA PANACEA)”, financed by the European Union’s DG ENV/MSFD 2020, under agreement No. 110661/2020/839628/SUB/ENV.C.2.
OSPAR, 2023. Pelagic Habitats Thematic Assessment. In: OSPAR, 2023: Quality Status Report 2023. OSPAR Commission, London. Available at: https://oap.ospar.org/en/ospar-assessments/quality-status-reports/qsr-2023/thematic-assessments/pelagic-habitats/