Epifauna and their importance in regeneration of the seabed. Dr. Sally Campbell
If you have ever wondered about the science of the seabed, read on. Dr Sally Campbell's article will help you to better understand the complex ecosystems that sustain the sea from below and will explain why fishing methods (such as dredging and bottom trawling) threaten the very viability of the marine environment as we know it...
Many years ago, as a graduate student, I had the good fortune to spend 3 weeks at Biologische Anstalt, a superb marine research station on Helgoland. This small island lies at the mouth of the Elbe in north Germany. The research project was to look at different marine deposits, their characteristics, epifauna and infauna. Infaunal species are those animals that live in the sediment, whilst epifaunal species are those that live on the surface of the seabed. Collectively this system is known as the benthos. Many epifaunal species are active and may move considerable distances. For this reason environmental monitoring often targets the infauna, as their presence at a site means that conditions are suitable for them over long periods. Most environmental monitoring uses benthic invertebrates, which include a wide range of polychaete worms, gastropod molluscs, bivalve molluscs, and various crustaceans.
Some of the most common epifauna are hydroids. Colonies of hydroids are typically 5 to 500 mm (0.2 to 20 inches) or more high and are branched; the branches bear the individuals, or zooids (hydroid polyps). Colonies of hydroids grow vegetatively by an increase in the number of hydranths. Reproductive polyps (gonozooids) occur intermittently on the colony. Most hydroids inhabit marine environments, although many of you may remember Hydra a solitary rather than colonial polyp, from biology sessions in school. There are about 2000 species of hydroids.
It had already been realised that the benthos was important in the development of productive ecosystems and so in Helgoland we looked at such seemingly mundane things as sand, gravel and mud grain size, water storing capacity, flushing and oxygen transfer from samples taken in and around the island and Elbe estuary. In addition the size and quantity of the infauna was measured by means of different mesh size of sieves, and the specimens preserved for identification. It was my first experience with infauna and I was amazed at the number of species, variety in size and the sheer density of these to be found in these microenvironments and the complexity of ecological systems.
In many ways paleoecology of marine rock strata has also informed us about the complex web of epifauna, so it is clear that the complex interrelationships in and on the sea bed are an important part of bioproductivity and have been present through geological time as demonstated in the fossil record. It is hardly surprising therefore that dredging and deep water trawling by damaging and destroying epifauna and infauna strike at the heart of ecological sustainability. The establishment of a dense epifaunal community depends on the habitat not being subject to excessive disturbance. Scallop dredging (for both Pecten maximus and Aequipecten opercularis) and other types of bottom fishing reduce habitat complexity by impacting sessile epifauna species, and by extension their associated organisms, and is now well documented (e.g. Bradshaw et al. 2000, 2001). Some of the most common epifauna are hydroids. They do not survive with dredging and bottom trawling as they are quite delicate in construction.
During the spring and early summer there is the peak growing/breeding season for many marine species. The warmer waters and increased phytoplankton create ideal conditions for reproduction and growth. Given that epifaunal hydroids are an important settlement substrate for scallop spat, no take zones (NTZs) create environments where spat settle and grow undisturbed especially in the early stages of their development. In NTZs, such as in Lamlash Bay (closed areas to dredging and bottom trawling), spat will have a greater density of hydroids on which to settle, and will be able to grow undisturbed for the whole of their lives. This is undoubtedly one reason why, 11 years after the closure of an area in the Isle of Man, scallops in the closed area are found in greater densities and are on average larger than their counterparts off Bradda Head, also the Isle of Man, which had no closure (Bradshaw et al. 2001).
Four sand and gravel extraction areas in the North Sea and English Channel were studied over a three-year period to determine the effect of differing levels of historic dredging activity on the nature of epifaunal recolonisation. (Smith et al. 2006). Diversity and abundance of epifaunal assemblages were generally lower at intensively dredged areas in comparison to those observed at nearby reference locations. Dominance of particular mobile epifaunal species was also recorded at the intensively dredged areas. Total biomass was also lower within previously dredged treatments at two extraction areas.
Sheppard (2006) reports that in many cases of dredging the destroyed, dominant colonies are centuries rather than mere decades old, and grew on antecedents which may be thousands of years old. Vast expanses of these have already been destroyed. The loss of the three-dimensional habitat that they provide to countless other species is likely to be as important as the loss of those framework species themselves, which means also that the unrecorded ‘by-catch’ and unseen ‘discard’ must be orders of magnitude greater than the targeted catch in terms of both biomass and diversity. The maerl beds in Lamlash Bay are the remnants of vast tracks of this fragile calcareous seaweed in the Clyde, and are thought to have been hundreds of years old in development.
Kaiser et al.(2006) found that the direct effects of different types of fishing gear were strongly habitat-specific. The most severe impact occurred in biogenic habitats in response to scallop-dredging. Analysis of the response of different feeding types to disturbance from fishing revealed that both deposit- and suspension-feeders were consistently vulnerable to scallop dredging across gravel, sand and mud habitats, while the response of these groups to beam-trawling was highly dependent upon habitat type. The biota of soft-sediment habitats, in particular muddy sands, were surprisingly vulnerable, with predicted recovery times measured in years. Slow-growing large-biomass biota such as sponges and soft corals took much longer to recover (up to 8 yr) than biota with shorter life-spans such as polychaetes (<1 yr).
Within the NTZ in Lamlash Bay, the research being carried out by Scottish Natural Heritage may begin to show that there is epifauna improvement in density and variety over time, and this in turn may provide an ideal environment for spat to settle. Although only a small area, lack of disturbance is the key to regeneration. Not just for hydroids and through them scallops, but also as nursery beds for white fish and many other marine species.
Work being done in California (Wethey et al. 2008) shows that marine sedimentary infauna alter the substrate, creating voids and air bubbles, manipulating grain and shell distributions, moving interstitial fluid and creating surface roughness elements. Worms and other burrowers continually change the characteristic of the sediments. They create permability and oxygen flow. The effect of pumping by Arenicola marina, Abarenicola pacifica, and Macoma nasuta affects the nutrient flux in sediments differing in permeability. So even though out of sight the infauna are an important part of the complex ecosystem that sustains epifauna on the sediment surface.
These results are often at the microlevel yet of extreme importance to the productivity of the inshore waters. It seems surprising therefore in the face of such well researched science that there is little control over dredging for scallops and bottom trawling for Nephrops in the Clyde. Many groups are calling for the return of the three mile limit for these forms of fishing in the Clyde. What is required is political will since the science is in place. As Poul Degnbol Head of the Advisory Programme at the International Council for the Exploration of the Sea (ICES) said in this year’s excellent Annual Lecture at the Scottish Association for Marine Science UHI, there are five elements in an Ecosystem Approach to Fisheries (EAF): these are to reduce fishing pressure to sustainable levels X 5. The will to change has to come politically first and foremost.
Bradshaw C, Veale L. O, Hill A. S, Brand A. R (2000) The effects of scallop dredging on gravelly sea-bed communities. In: Kaiser MJ, de Groot SJ (eds) Effects of fishing on non-target species and habitats. Blackwell Science, Oxford, pp 83–104
Bradshaw C, Veale L. O, Hill A. S, Brand A. R (2001) The effect of scallop dredging on Irish Sea benthos: experiments using a closed area. Hydrobiologia 465:129–138
Kaiser M J, Clarke K. R, Hinz H, Austen M. C. V, Somerfield P.J, Karakassis I (2006) Mar Ecol Prog Ser. 311, 1–14
Sheppard C (2006) Editorial: Trawling the sea bed. Marine Pollution Bulletin 52, 831–835
Smith R, Boyd S.E, Rees H.L, Dearnaley M.P, and Stevenson J. (2006) Effects of dredging activity on epifaunal communities: surveys following cessation of dredging. Estuarine Coastal and Shelf Science, 70(1-2): 207-223
Wethey D. S, Woodin S. A, Volkenborn N, Reise K. (2008) Porewater advection by hydraulic activities of lugworms, Arenicola marina: A field, laboratory and modeling study. Journal of Marine Research 66: 255-273.