Condition of Benthic Habitat Communities: Assessment of some Coastal Habitats in Relation to Nutrient and/or Organic Enrichment

Coastal waters represent the interface between land and ocean and are defined under the European Union Water Framework Directive (WFD) as extending one nautical mile from a baseline defined by the land points where territorial waters are measured. ‘Water bodies’ are defined by European Union Member States by sub-dividing these coastal waters according to their hydrological characteristics. Some coastal waters of OSPAR Contracting Parties have, to a varying degree, been affected by nutrient and/or organic enrichment (either as a direct or indirect effect of nutrient inputs or of organic enrichment) and this may be a cause of nuisance and toxic algal blooms, loss of benthic vegetation due to shading, and modification of benthic fauna communities due to specific sensitivities.

Detailed monitoring and assessment methods are provided in annex 2 of the BH2 CEMP guidelines. 

The biological quality elements required for the integrated assessment of ecological status (European Union Water Framework Directive (WFD), Annex V) of coastal waters are phytoplankton, macroalgae, angiosperms and benthic fauna (European Parliament, 2000; Solheim et al., 2012).

The WFD requires that standardised methods are used for monitoring biological quality elements and that the good status class boundaries for each biological quality element are intercalibrated across EU Member States sharing similar types of water bodies (Neto et al., 2018; Wilkes et al., 2018; Van Hoey et al., 2019; Salas Herreroet al., 2020).

The WFD requires that the overall ecological status of a water body is determined by the results for the biological or physicochemical quality element with the worst class determined by any of the biological quality elements. This is called the ‘one out - all out’ principle. The rationale for this is to avoid averaging the impacts on different quality elements due to different pressures and therefore overlook some significant pressures, and also to provide sufficient protection for the most sensitive quality element to significant pressure.

By definition, the WFD aims to assess the “water quality” of water bodies through quality elements such as benthic invertebrates or macrophytes. Benthic habitat communities are thus mainly assessed for the WFD to reflect the main pressures in the coastal zone, such as direct or indirect effects of nutrient and/or organic enrichment as a result of nutrients inputs from land-based sources and through riverine inputs. Contaminants are generally assessed in sediment. For these reasons, WFD monitoring and related indices were generally designed to focus on the effects of nutrient and/or organic enrichment, and other pressure types are not directly or not at all assessed (Poikane et al.,2020). However, this does not mean that the indices used are not themselves sensitive to other pressure types. Currently several statistical indices that are used, notably those including a species sensitivity classification, have been developed and calibrated to assess nutrient and/or organic enrichment. The European Union Marine Strategy Framework Directive (MSFD) requires a reconsideration and adaptation of monitoring, to consider and assess other pressure types (both inshore and offshore; Poikane et al., 2020; Salas Herrero et al., 2020).

Diversity indices and species richness indices, as well as sensitivity / tolerance species classification systems, have long been used to assess the qualitative state of benthic habitat communities. The development of Benthic Indices (BI), combining these indices and classifications, was made mandatory by the WFD. Species composition and relative abundance of the benthic habitat community are basic common metrics. Multi-metric BI classically contain diversity indices, species richness indices and / or the proportions of sensitive, tolerant and opportunistic species. Species sensitivity is a proxy used to quantify disturbance for some pressures on a benthic habitat community: either as a direct or indirect effect of nutrient or organic enrichment, oxygen depletion or hydrological changes. This requires a good knowledge (for the communities and species assessed) of natural spatial and temporal scales of variation and sensitivity to a specific pressure gradient.

Multi-dimensional deviation in community structure from a reference condition, aims to link pressure (for management issues) and pressure-impact calibration of indices (level of disturbance / resilience). This is set up with various pressures types at the community level, by combining biological and pressure data. Analysis of sensitivity is then elaborated at community level rather than species level. This is an important distinction and a point of attention for further development by experts and implementation for management issues (i.e., specific parameters / metrics to be monitored). These principles, commonly agreed at OSPAR level, are detailed and illustrated in OSPAR (2017a, 2018).

The first OSPAR assessment results (OSPAR 2017b; McQuatters-Gollop et al., 2022) or the Biological Quality Elements ‘Benthic Invertebrates’ and ‘Macroalgae and Angiosperms’ in coastal waters were requested through a formal data call to OSPAR Contracting Parties and notably WFD national contacts. In addition, information on the indices used and the monitoring stations in coastal water bodies was requested. Information on the methodology of the various benthic indices (sampling strategy, data evaluation, reference conditions, detected pressures) were mainly derived from the WISER methods database and intercalibration reports (Birk et al., 2010; Van Hoey et al., 2015). This assessment was done for three of the five OSPAR regions.

The current (2023) OSPAR assessment data (WFD assessment results) were obtained directly from the WISE Water Framework Directive Database, in early 2022, which includes available national information under WFD 2010 to 2015 reporting cycle (as reported in 2016) as well as data from the previous WFD cycle, as reported in 2010. A formal data call was done by OSPAR to encourage Contracting Parties to update or make their data available under the WISE database. Data for the next (2016 to 2021) reporting cycle were not available from any Contracting Party.

The data about coastal water bodies and monitoring stations were uploaded from WISE WFD reference spatial data sets and, for the UK, from official reporting sites (England, Northern Ireland, Wales, Scotland).

Information on the methodology of the various benthic indices was mainly derived from intercalibration reports (Birket al., 2010; Van Hoey et al., 2015; Neto et al., 2018; Wilkes et al., 2018; Van Hoey et al., 2019; Salas Herrero et al., 2020). The intercalibration of the ecological quality status and assessment methods were carried out for benthic invertebrates (Van Hoey et al., 2019), opportunistic macroalgae (Wilkes et al., 2018) and seagrasses (Neto et al., 2018) in the coastal waters of the North-East Atlantic. As a result, the values of the boundaries between classes of Ecological Quality Ratio of the Member States were established (European Commission, 2018).

In 2022, WFD assessment results for benthic invertebrates were provided from all OSPAR Contracting Parties’ coastal water bodies, in the whole OSPAR maritime area, but published only for three OSPAR Regions (II – Greater North Sea, III – Celtic Seas, IV – Bay of Biscay and Iberian Coast) where this indicator was adopted as common (Table a). The assessment results for macroalgae, angiosperms and other flora were available from nine countries. For the Netherlands and Belgium, this quality element is not present, thus not relevant nor assessed in their coastal waters.
 

Table a: Coastal water bodies’ WFD quality status national availabilities, per quality element for the reporting years 2010 and 2016 (from the River Basin Management Plan database).

Detailed assessment methods and intercalibration exercises are available at:
https://ec.europa.eu/environment/water/water-framework/facts_figures/guidance_docs_en.htm
https://www.eea.europa.eu/themes/water/european-waters/water-quality-and-water-assessment/water-assessments
Details on metrics and indices used by Contracting Parties to assess and provide results for the WFD are described in the annex 2 of the BH2 CEMP guidelines.

Sub-regional assessment outcomes and locally impacted areas, according to available data at the time of writing, are presented in Figure a and Figure b.

All Contracting Parties from OSPAR Regions II, III and IV reported to the WISE database, through the WFD engagement for European Union Member States, or voluntarily for others.

Confidence Assessment:

The assessment methodology has been applied over successive WFD cycles. However, the inter-calibration of results between EU Member States is not fully solved and therefore underpins moderate confidence. Some regional and local variations of the quality status assessment may be biased by the use of different indices or monitoring/assessment methods.

For the data, there is generally quite good coverage, but with many gaps in several areas, for most assessment units. There was no information available, in the time and resources dedicated to this assessment, to gather details on the reasons for these gaps, and the potential bias implied in proportional results. Therefore, there is moderate confidence in the data for this assessment, at a sub-regional level.

This assessment focused on the status of coastal habitats in relation to nutrient and/or organic enrichment, as one component of a more comprehensive indicator of the condition of benthic habitat defining communities (Condition of Benthic Habitat Communities) and as part of the OSPAR North-East Atlantic Benthic Habitat Thematic Assessment.

Most OSPAR Contracting Parties undertake large-scale marine benthic monitoring, at least for the European Union Water Framework Directive (WFD) and other national programmes. Finer-scale networks of state-pressure relationship assessment areas are more heterogeneous and should be further investigated. Developing coordinated monitoring (or even better, joint monitoring) would be more cost-efficient and would ensure coherence and robustness for an assessment at (sub)regional scale.

Each country currently stores its monitoring data and common methodology (and tools), but improvement to achieve coherence and data availability is still needed. Development of data flow arrangements to access and analyse data has improved since 2017, but heterogeneities between indicators and assessment methods limits inter-calibration and comparison of results at wider regional and European scales. The need for these steps should be anticipated and relevant work should be coordinated at a (sub)-regional scale to ensure coherence and facilitate the data flow for this OSPAR specific assessment to be used also for EU Member States’ reporting requirements (WFD, MSFD and HD).

By targeting an increased coherence and complementarity between the different indices used, the development of methods, monitoring and data flow to assess other pressures types, and ultimately the cumulative effect of pressures, should facilitate interpretation of results and guidance for targeted response measures (notably through Regional Seas Conventions and EU policies; see Carvalho et al., 2019; Lizińska & Guérin 2022). Until then and after, the use of experts’ judgment is recommended with adapted methods and scales (McQuatters-Gollop et al., 2022)

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