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Boreal environment research 16 (suppl. a): 209–219 © 2011 issn 1239-6095 (print) issn 1797-2469 (online) helsinki 31 march 2011 effect of physical disturbance on the soft sediment benthic macrophyte and invertebrate community in the northern Baltic sea Kristjan herkül1)2), Jonne Kotta1) and merli Pärnoja1) 1) Estonian Marine Institute, University of Tartu, Mäealuse 14, EE-12618 Tallinn, Estonia 2) Institute of Ecology and Earth Sciences, University of Tartu, Vanemuise 46, EE-51014 Tartu, Estonia (corresponding author’s e-mail: [email protected]) Received 20 Nov. 2009, accepted 9 June 2010 (Editor in charge of this article: Johanna Mattila) herkül, K., Kotta, J. & Pärnoja, m. 2011: effect of physical disturbance on the soft sediment benthic mac- rophyte and invertebrate community in the northern Baltic sea. Boreal Env. Res. 16 (suppl. a): 209–219. Strong storms and ice scour are the most severe physical disturbances in the shallow water areas of the northern Baltic Sea. We studied experimentally the effects a physical distur- bance — such as the removal of the surface sediment layer, vegetation, and benthic inver- tebrates and the timing of this disturbance (spring, summer) — had on the development of soft bottom macrophyte and invertebrate communities in a shallow bay. Disturbance had an immediate effect on the community in spring but not in summer. The lack of significant immediate effects in summer was attributed to a drifting algal mat that quickly introduced most of the local species to the newly established experimental plots. The springtime dis- turbance reduced the species richness and total biomass of phytobenthos in summer; the effect, however, was not detectable by autumn. Disturbance in spring decreased the total abundance and biomass of zoobenthos in autumn but not in summer. A summertime distur- bance had no effect on the autumn benthic communities. Introduction bottom communities (e.g. Rumohr et al. 1996, Kaiser et al. 2000, Powilleit et al. 2006, Smith Disturbance is a key factor regulating the struc- et al. 2006). Currently, there are only a few stud- ture and functioning of natural communities ies on the effects of mechanical disturbance on (Sousa 1984, Whitlatch et al. 1998, Zajac et al. nontidal vegetated soft bottom communities (e.g. 1998, Widdicombe and Austen 2001, Dernie et Boström and Bonsdorff 2000, Herkül and Kotta al. 2003). Numerous studies on physical distur- 2009). Although macrophytes provide both habi- bances in marine environments focused either tat and food for a variety of benthic invertebrates on intertidal systems (e.g. Kim and DeWreede in such communities, it is not uncommon that 1996, Hall and Harding 1997, Keough and disturbance experiments exclude macrophytes. Quinn 1998, Ramage and Schiel 1999, Cowie et Studying both benthic macrovegetation and al. 2000, Boese 2002, Rossi et al. 2007, Schiel invertebrates allow us to demonstrate the links and Lilley 2007), rocky subtidal (Wernberg and between disturbance, macrophytes, and inverte- Connell 2008), or subtidal unvegetated soft brates. 210 Herkül et al. • Boreal env. res. Vol. 16 (suppl. a) Coastal benthic communities in boreal envi- this is expected to be especially relevant in the ronments are exposed to natural physical dis- Baltic Sea. Species diversity is low in the north- turbances of varying magnitude caused mainly ern Baltic Sea and one functional group is often by storm events and ice scour. In sedimentary represented by a few or a single species (Kiirikki habitats, physical disturbance modifies sediment 1996, Bonsdorff and Pearson 1999, Bonsdorff structure and seriously damages infauna and 2006). Therefore, it is expected that physical macrophytes and to a lesser extent epifauna disturbances may pose an additional challenge (Boström and Bonsdorff 2000, Dernie et al. for the Baltic communities due to the presence 2003). As invertebrates show a high degree of of other stress factors such as low salinity and selectivity for a sediment structure and macro- large temperature fluctuations (Segerstråle 1957, phyte species (Kotta and Orav 2001, Orav-Kotta Kotta et al. 2008a). However, the Baltic species and Kotta 2003, 2004), physical disturbance may are tolerant to strong fluctuations in the physical also indirectly shift the structure and function- environment (Bonsdorff 2006, Powilleit et al. ing of benthic communities. Disturbance causes 2006, Kotta et al. 2009a) and thus it is likely that a partial or total removal of dominant species they can easily cope with physical disturbance. creating unoccupied space for further coloniza- Shallow water vegetated soft bottom commu- tion and may thus alter the dominance structure nities are widespread in the northern Baltic Sea (Sousa 1984, Schiel and Lilley 2007). Strong and have an important role in the coastal ecosys- levels of physical disturbance favours the domi- tems. These communities, dominated by phan- nance of opportunistic fast growing species and erogams (e.g. Potamogeton spp., Zannichellia mobile epifauna (Sousa 1984, Pugh & Daven- palustris) or charophytes, form an important port 1997, van Dalfsen et al. 2000, Posey and habitat for a variety of benthic invertebrates Alphin 2002). and macrophytes (e.g. Boström and Bonsdorff The coastal ecosystems of the northern Baltic 1997, Appelgren and Mattila 2005, Hansen et Sea are very dynamic and characterised by al. 2008). Vegetated shallow water soft bottoms high physical disturbance (Hällfors et al. 1981, generally provide feeding and nursery areas for Bonsdorff 2006). Similarly to other boreal eco- several fish and bird species (Mattila et al. 1999, systems, strong storm events and ice scour are Grenouillet and Pont 2001, Heck et al. 2003, regarded as the most severe natural physical dis- Sandström et al. 2005, Schmieder et al. 2006). turbances in shallow water areas (Hällfors et al. Despite their importance, the role of physical 1981, Kiirikki 1996, Idestam-Almquist 2000). disturbance on the development of such soft In addition to natural physical disturbances, bottom communities remains largely unevalu- anthropogenic disturbances like boating and ated. There is evidence that, concurrent with dredging activities and resource extraction are the climate change, storms may become more common in coastal areas (Sandström et al. 2005, frequent and violent (Woth et al. 2006). Conse- Szymelfenig et al. 2006, Kotta et al. 2009a). The quently, mechanical disturbance may become magnitude of mechanical disturbance in shallow more frequent and severer and thus have a water soft bottom communities in the northern greater effect on benthic communities. Baltic Sea may range from a small impact that In this paper, we studied the effects of physi- removes a few individuals to a total removal of cal disturbance (mimicking an ice scour or a a community caused by severe ice scour. The severe storm) and the timing of disturbance timing of disturbance is known to determine (spring, summer) on the development of soft the nature of the effects on benthic communi- bottom macrovegetation and invertebrate com- ties (Sousa 1984, Benedetti-Cecchi and Cinelli munities in a shallow bay in the northern Baltic 1994, Skilleter et al. 2006); e.g. the effect of a Sea. We hypothesize that (1) disturbance reduces disturbance on a benthic community depends the number of benthic species, (2) disturbance on the phase of macrobenthic seasonal succes- changes the abundances and biomasses of ben- sion in which the disturbance takes place (Kim thic species, and (3) the magnitude of impacts and DeWreede 1996). Due to strong seasonality, depend on the timing of disturbance. Boreal env. res. Vol. 16 (suppl. a) • Effect of physical disturbance on benthos 211 Material and methods lished in 20 July 2005 were sampled on 20 Sep. 2005. All comparisons for assessing the impact Study site of disturbance were made between the disturbed and control quadrates sampled on the same date. The study was conducted in Kõiguste Bay An Ekman type bottom grab sampler (0.02 m2) (58°22.10´N 22°58.69´E), in the Gulf of Riga, was used for sampling benthos. The grab sampler in the northern Baltic Sea. Kõiguste Bay is a was manually operated by a diver to ensure rep- brackish, nontidal, semi-enclosed and shallow resentativness of samples. Benthos samples were water basin. The prevailing sediment types in the sieved through a 0.25 mm mesh and the residuals bay are sandy clay mixed with pebbles, gravel, were placed in plastic bags. The samples were or boulders. The area is influenced by a dif- stored deep frozen (–18 °C) until analysis. In the fuse nutrient load from the moderately eutrophi- laboratory, all samples were sorted under a bin- cated Gulf of Riga. The experiment was carried ocular microscope (20–40¥ magnification). All out in a shallow water area (1 m) where the macrobenthic species were identified to the spe- bottom sediment was characterised by a layer cies level, except for oligochaetes, chironomids, of sand mixed with some pebbles on hard clay. and juveniles (size < 5 mm) of gammarid amphi- The phytobenthic community was dominated pods. Individuals of all taxa were counted and by the higher plant Potamogeton pectinatus. weighed. Prior to weighing, animals and plants Several green, brown, and red algal species like were dried at 60 °C for 48 hours and two weeks, Cladophora glomerata, Pilayella littoralis, and respectively. Abundances and biomasses were Ceramium tenuicorne grew on higher plants and calculated per square metre. stones. The prevailing benthic invertebrates in Sediment samples for organic matter content the experimental area were the cockle Cerasto- were collected from quadrates using a cylindri- derma glaucum, the gastropods Hydrobia ulvae cal core ( 1.6 cm). Care was taken to avoid and Theodoxus fluviatilis, the polychaete Hediste trapping animals and plants in the samples. The diversicolor, and chironomid larvae. organic matter content was measured as a per- centage loss of weight on ignition (500 °C, 3 h) of dry sediment (60 °C, 7 days). Experimental design and sampling The mechanical disturbance involved the removal Statistical methods of the upper sediment layer (ca.

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