Threats to cold water coral reefs, sand banks and seagrass habitats in the North Sea by climate change effects

Abstract

According to the IPCC research in the field of climate change over the past decades shows that the observed changes in regional climate have affected many physical and biological systems, and there are preliminary indications that social and economic systems have been affected.

Moreover, those changes in climate could increase the risk of abrupt and non-linear changes in many ecosystems which would affect their function, biodiversity and productivity.

Very sensitive to the processes taking place tend to be coastal areas, where at the same time reach variety of species can be observed. That is why such areas are the first to experience the effects of sea level rise and other climate change consequences.

Key words

climate change, coral reefs, sand banks, seagrass meadows, temperature rise, storm surges, risks, sea level rise

Introduction

About 70% of the Earth surface is covered by the ocean. A high diversity of life forms can be found there. The importance of the world’s oceans increases as their role in determining global climate is appreciated along with the examples of highly diverse ecosystems such as coral reefs.

Coastal zones have the highest concentration of natural hazards in the world. The coastal zone commonly has the following hazards: coastal erosion, landslides, river or estuary flooding, storm surge flooding and winds from ocean borne storm events (e.g. hurricanes, cyclones, and typhoons), earthquakes, tsunamis, and volcanic eruptions .

Moreover, climate change is the enhancing factor for all those threats mentioned above under the circumstances of sea level rise and temperature rise. So, the Intergovernmental Panel on Climate Change (IPCC) predicts that, by 2100, climate change might lead to an increase in the surface air temperature of the Northeast Atlantic by 1,5°C and a rise in sea level by 25 cm to 95 cm.

Understanding how the climate change can impact vulnerable ecosystems of cold water coral reefs, sandbanks or seagrass beds is therefore of growing importance.

North Sea habitat

Habitat is a term that is often difficult to describe exactly because there are different perspectives on its definition.

Habitat is generally thought of as a place where an organism is found, such as estuaries, salt marsh, seagrass, and cobble fields . Describing habitat is complicated by issues of scale and complexities in natural resources. Right whale habitat is described in terms of oceans (1000s km), while juvenile fish habitat is described by unique seafloor characteristics or microhabitats (cm to m) .

Sandbanks

Sandbanks are sandy ridges that clearly rise above their surroundings; according to the Habitats Directive Annex definition they must be permanently submerged. Substrate is primarily a sand surface to a gravel mix; minimum thickness of 30 to 40cm must be present to provide habitats for typical sandy bottom communities. Vegetation is often absent, or it is very scarcely presented (e.g. macrophyte vegetation).

Scientists have verified the presence of sandbanks worthy of protection within the Exclusive Economic Zone (EEZ) of the North Sea. Particularly large sandbanks are the Dogger Bank in the North Sea, and a smaller one the Amrum Outer Bank (Sylter Außenriff) [1].

Important ecological functions of sandbanks:

• feeding habitat for resting and wintering birds, e.g. sea ducks (incl. Common scoter) and loons (Black-throated diver and Red-throated diver);
• feeding and nursery grounds for many fish species;
• regeneration and recolonization reservoir for deeper marine areas after catastrophic oxygen depletion events or iced winters in nearshore shallow areas;
• stepping-stone function for the expansion of bottom organisms throughout the North Sea (e.g. Dogger Bank) [2].

Reefs

Reefs are hard mineral substrates such as rocks, till, or stones, primarily moraine ridges with block and stone cover in gravel/sandy surroundings, which rise mildly to prominently above the seafloor;

• biogenic hard substrates such as honeycomb (Sabellaria) reefs and mussel banks are present;
• they are permanently submerged;
• often they are overgrown with mussels and a characteristic macrofauna.

Unlike tropical corals, cold water corals lack photosymbiontic algae in their tissue. However they feed by catching particles out of the surrounding seawater with their tentacles .

The scientists found such reefs and reef-like structures in many areas of the North Sea. In the North Sea, regions proving to be of particular ecological value include the areas of the Borkum Reef Ground, the eastern slope of the Ancient Elbe River Delta, and the Helgoland Stone Ground, not to mention the Helgoland bedrock, which is unique in the southern North Sea [3]

More than 160 macrozoobenthic (bottom-dwelling animal) species were verified on the Borkum Reef Ground, including over 20 Red List species. The interlacing of small-scale reefs with the sandbank habitat leads to an especially high species diversity here, with numerous typical benthic biocoenoses.

In addition, the Borkum Reef Ground is an example of a sandbank with stone fields or stony/gravelly areas as reef-like structures. This substrate diversity provides a variety of habitats and, in turn, a corresponding wide spectrum of species.

They are the habitat for a large number of species of sessile creatures such as sea squirts, anemones, sea anemones, bryozoans, mussels and many species of sea worms.

Important ecological functions of reefs:

• feeding grounds for birds and marine mammals;
• habitat and retreat;
• habitat and nursery area with high species diversity;
• spawning grounds and feeding ground for fish;
• stepping-stone and regeneration reservoir for the expansion of benthic organisms.

Biological diversity associated with the reefs is around three times as high as that of the surrounding soft sediment seabed, indicating that these reefs create biodiversity hotspots and increased densities of associated species . This includes not only several hundred species of coral, but thousands of fish and invertebrate species such as sponges, crabs, shrimps, lobsters, sea anemones, worms, sea stars and sea urchins, octopuses, squid, snails and nudibranchs.

Seagrass meadows

Eelgrass (Zostera marina) Credit: NOAA

Seagrass is a flowering plant that lives in a marine or brackish environment. Seagrasses are sometimes found in patches, but these patches can expand to form huge seagrass beds, or meadows. The beds can be made up of one species of seagrass, or multiple species.

Seagrasses require a lot of light, so the depths at which they occur in the ocean are limited by light availability. Seagrasses are found in protected coastal waters such as bays, lagoons, and estuaries.

Seagrasses attach to the ocean bottom by thick roots and rhizomes, horizontal stems with shoots pointing upward and roots pointing downward.

Important ecological functions of seagrasses:

• provision of important habitat to a number of organisms;
• nursery areas;
• animals such as manatees and sea turtles feed on animals that live in the seagrass beds;
• roots help stabilize the ocean bottom;
• help with water clarity by trapping sediments and small particles in the water column, and help boost local economies through supporting vibrant recreation opportunities.

Organisms that make the seagrass community their home include bacteria, fungi, algae; invertebrates such as conch, sea stars, sea cucumbers, corals, shrimp and lobsters; a variety of fish species including snapper, parrotfish, rays, and sharks; seabirds such as pelicans, cormorants and herons; sea turtles; and marine mammals such as manatees and bottlenose dolphins.

Impacts of climate change on the habitats

Some climate change models predict that, within this century, average global temperatures will increase by over 1.5 degrees and sea levels will rise by over 2 meters. Other models forecast even higher values with an increased frequency and intensity of excessive precipitation events and of violent land and sea-based storms

Lovejoy and Hannah state that not all changes (calcium carbonate saturation rate, sea level and temperature) are likely to have negative impact on marine biodiversity, but the combination of these changes is expected to be the major driver on the distribution and abundance of marine organisms.

Generally, the following effects of climate change to the habitats are identified :

• gain & loss of specific habitats & species;
• “upward” shift of tidal gradients in habitats (depending on nature of borders and rate of sea level rise);
• temperature rise (shift to more cold winters and more hot summers);
• mortality rates increasing (freezing, anoxia, metabolic imbalance);
• storms and waves cause resuspension, so from benthic primary production to pelagic primary production
• more rainfall cause water change to more brackish (shifts from marine to brackish flora and fauna);
• loss of ecosystem engineers (e.g., shift from sedimentation to erosion);
• loss of strong filter-feeders (e.g., less grazing on phytoplankton & larvae);
• loss of biodiversity;
• competitive abilities change (e.g., edible mussels vs. inedible oysters);
• toxicity (e.g., bloom of Verrucophora verruculosa);
• predation pressure (e.g., occurrence of Mnemiopsis leidyi).

Moreover, sea level change leads to further coastal squeeze, resulting from a steepening beach profile and foreshortening of the seaward zones. .

Particular threats to seagrasses: among others can be named:

• storm surges,
• floods and droughts affecting water salinity,
• disruption of seagrasses by small predators as they search for food, especially driven by sea level rise or changing consumption pattern due to temperature change,
• CO2 concentration change with UV increase, as well.

Threats to reefs: greatly impacted by humans, impact of natural threats is enhanced by:

• severe weather events (as prolonged rain);
• predation pattern change;
• carbon dioxide results concentration result in corals’ weaker skeletons, making them more vulnerable to their environment and human impacts.

Threats to sandbanks: particular impacts are driven by severe weather events, when the seabed pattern changes, causing vulnerable situation for inhabiting microorganisms.

Discussion of approaches to deal with the problem

Continuing long-term field observations help scientists to build scenarios and work out models to deal with changes in:

• temperature & salinity;
• currents & wave heights;
• nutrients concentrations;
• phytoplankton;
• macrozoobenthos composition;
• fish, birds & sea mammals species communities.

Still missing parameters (uncertainties on climate change models) can influence the results. Therefore appropriate spatial and temporal resolutions play an important role to receive up-to-date results.

Tools used could be as well:

• gathering new information and reports’ consulting (e.g., quality status reports);
• long-term field observations (e.g., TMAP);
• use of satellite images.

In that case realistic scenarios can be worked out, to be applied further for management strategies and continued research activities.

Conclusions

Global temperatures and carbon dioxide concentrations are now higher than they have been for at least the past 400.000 years, according to IPCC . According to Lovejoy and Hannah it is important to understand how biological systems will respond to these changes, because they are likely to seriously affect the world’s oceans. Therefore, further research as well as monitoring and cooperation of different stakeholders are important.

See also

Threats to the coastal zone

North Sea

Seagrass meadows

Seagrass recovery and restoration in the Wadden Sea

Nature-based shore protection

Dynamics, threats and management of biogenic reefs

Case study risk analysis of marine activities in the Belgian part of the North Sea

Further reading

• Nicholls, R., Klein, R. and Tol, R. 2007. Managing coastal vulnerability. Elsevier Ltd. Chapter 13 (p. 221).
• Clark, J. (editor) 1992. Integrated management of coastal zones – FAO Fisheries technical paper 327.
• Sorensen, J. (editor) 2000. Baseline Background report. Harbour and Castal Center, Urban Harbours Institute, University of Massachusets, Boston.
• http://www.algaebase.org

References

The main author of this article is Tanya From Please note that others may also have edited the contents of this article. Citation: Tanya From (2023): Threats to cold water coral reefs, sand banks and seagrass habitats in the North Sea by climate change effects. Available from http://www.coastalwiki.org/wiki/Threats_to_cold_water_coral_reefs,_sand_banks_and_seagrass_habitats_in_the_North_Sea_by_climate_change_effects [accessed on 28-11-2024] For other articles by this author see Category:Articles by Tanya From For an overview of contributions by this author see Special:Contributions/Tatianaf