An experimental study of plant habitat choices by macroinvertebrates in brackish soft-bottom bays by Hanna Axemar Supervisors: Joakim Hansen & Lena Kautsky Plants & Ecology Plant Ecology 2007 /9 Department of Botany Stockholm University Plants & Ecology Plant Ecology Department of Botany Stockholm University S-106 91 Stockholm Sweden © Plant Ecology ISSN 1651-9248 Printed by Solna Printcenter Cover: Myriophyllum spicatum with epiphytes in a shallow soft-bottom bay. Photo by Joakim Hansen. 2 Summary During the last decade there has been an increased research focus on shallow soft-bottom bays in the Baltic Sea. This study examined animal and plant interactions in this environment, which previously has been scantly studied. The habitat preference of four macroinvertebrates between three different aquatic plants was studied in experiments on the Askö laboratory in July 2006. The aim of the study was to examine if macroinvertebrates have any habitat preference between plant species and between single and multi-species habitats. The study was conducted with pair-wise comparisons of the habitats. A total of four different habitats were planted; three habitats of a single plant species and one habitat consisting of all three species. The hypotheses were that a single species habitat with a more complex plant structure will be preferred over a less complex by macroinvertebrates and that a heterogeneous multi- species habitat will be preferred by over a homogeneous single-species habitat. The most significant result showed that the amphipod Gammarus spp. clearly preferred a habitat consisting of Myriophyllum spicatum, with its delicate and complex structure, and the habitat with all three plants combined; M. spicatum, Potamogeton pectinatus and Chara baltica. The isopod Idotea spp. made a similar choice, however not significant. The habitat preference of the gastropod Theodoxus fluviatilis was not significant, but this species had a trend of preferring P. pectinatus. The gastropod Bithynia tentatculata did not make any active habitat choice. Sammanfattning Det är först under det senaste decenniet som forskning om Östersjöns grunda mjukbottenvikar har kommit i fokus. Den här studien undersökte interaktioner mellan djur och växter i dessa miljöer, något som tidigare ringa studerats. Fyra makroevertebraters preferens av växthabitat bestående av tre olika vattenväxter studerades i experiment på Askölaboratoriet i juli 2006. Syftet var att ta reda på om makroevertebrater har någon habitatpreferens mellan olika växtarter, samt mellan enarts- eller flerartshabitat. Studien genomfördes med parvisa jämförelser mellan habitaten. Totalt planterades fyra olika habitat; tre enartshabitat och ett habitat bestående av alla tre växtarter. Hypoteserna var att ett enartshabitat med en mer komplex växtstruktur föredras framför ett mindre komplext habitat av makroevertebrater, samt att ett heterogent flerartshabitat föredras framför ett homogent enartshabitat. Resultaten visade att de olika djuren har olika habitat preferens. Det mest signifikanta resultatet visade att märlkräftan Gammarus spp. hade en klar preferens för både axslinga Myriophyllum spicatum, med sin fina komplexa struktur, samt habitatet bestående av alla tre växter; M. spicatum, borstnate Potamogeton pectinatus och grönsträfse Chara baltica. Tånggråsuggan Idotea spp. visade tendens på ett liknande val, dock utan signifikans. Östersjöbåtsnäckan Theodoxus fluviatilis gjorde inte något signifikant habitatval, men visade en tendens att fördra P. pectinatus. Bithyniasnäckan Bithynia tenatculata gjorde inte något aktivt habitatval. 3 Introduction The Baltic Sea is a brackish and geologically young sea. The flora of the Baltic Sea consists of both marine and freshwater species (Snoeijs 1999). In the northern Europe there is a continuous isostatic land rise, which results in a succession of coastal bays that are slowly isolated from the sea. These shallow, soft-bottom, wind and wave-protected environments have a rich flora community consisting mainly of freshwater angiosperms and charophytes (Munsterhjelm 2005). Submersed plants can affect the associated aquatic fauna community in different ways. The structure of the plant is important for the macrofauna by both offering the animals shelter to avoid predators or to help the predator lurking on its prey. Plant species that have a morphological structure with many branches and delicate leaves have been observed to house a rich macroinvertebrate fauna community (McAbendroth et al. 2005; Humphries 1996; Xie et al. 2006). Furthermore, a larger leaf area is likely to contain a higher density of epibionts, and thereby offers more food. The aquatic plants could also be a direct food source for several macrofauna species (e.g. Bodström & Mattilla 2005, Kornijow et al. 1995, Nicotri 1980, Hartvig & Kraufvelin 2004, Kotta et al. 2004). Moreover, the whole composition of the flora community can influence the fauna. A heterogenic community, containing several species of plants, may theoretically house a more diverse fauna by offering an addition of niches in comparison to a homogenic plant community (Statzner & Moss 2004). Disturbance, like eutrophication, can alter interactions between organisms in an ecosystem. Enhanced levels of nutrients in coastal bays have the consequence of favouring some plants more than others, which results in changes in the plant community (Schramm & Nienhuis 1996). In eutrophied shallow sheltered coastal bays of the Baltic Sea charophyte populations have declined and been replaced by angiosperm species more tolerant to turbid conditions, e.g. Ceratophyllum demersum, Myriophyllum spicatum and Potamogeton pectinatus (Munsterhjelm 2005). These changes can in turn affect the fauna community in the bays. Other forms of disturbance, like introduced (non-native) species, pollution, boat traffic and dredging may in a similar way affect the plant and animal community (Eriksson et al. 2004, Schramm & Nienhuis 1996, Cheruvelil et al. 2002). For example Eriksson et al. (2004) reported a decline of flora species in shallow areas of the Baltic Sea with excessive boat traffic. 4 The research on shallow soft-bottom bays of the Baltic Sea was until the 1990´s limited, but has during the last years received more focus. Surveys of the submersed vegetation and fish community have been conducted in several areas (e.g. Dahlgren 2002, Andersson 2000, Persson & Schreiber 2004, Persson & Johansson 2006, Edlund & Siljeholm 2003). Many of these studies have been conducted within the framework of the EU habitat and water directive. However, there are few studies on the macroinvertebrate fauna community in vegetated shallow soft-bottom bays of the Baltic Sea. The aim of this study was to see if there were any difference in the habitat preference among macroinvertebrates between three submerged plants; Myriophyllum spicatum, Potamogeton pectinatus and Chara baltica. The plants were chosen as they differ in tolerance to eutrophication, differ in structure and possess different levels of allelochemistry. A further aim was to test the effect of plant diversity on the habitat choice by the macroinvertebrates comparing a single species habitat with a three species habitat. For the study four common invertebrates in shallow soft-bottom bays of the Baltic Sea were used; i.e. Idotea spp, Gammarus spp, Bithynia tentaculata and Theodoxus fluviatilis. The following hypotheses were tested; I.) A habitat with a more complex plant structure will be preferred by the individual animal taxon compared to a less complex plant structure. II.) A heterogenic habitat consisting of three plant species will be preferred by the individual animal taxon compared to a homogenic one plant species habitat. Material and method The study was conducted in July 2006 at the Askö laboratory, situated in the Trosa archipelago (N58º 49' E17º 38'). The habitat choice of the fauna species were studied in pair- wise sets of flora species and for these plastic boxes where used (5.58 * 3.6 * 2.8 dm on the inside). In each end of these, two different habitats were created consisting of one of the three aquatic plant species, M. spicatum, P. pectinatus and C. baltica, or a mixture between the 5 three of them. A layer of 4-7 cm of sand covered the bottom of each box. The sand was collected at the southeast part of the island (Fig.1) and filtered through a 5 mm sieve. During the experiment the containers were filled with about 50 L of filtered seawater. A. B. C. Figure 1. A map over Askö island in the Trosa archipelago. (A) Askö laboratory where the study was conducted (N58º 49' E17º 38'). The plants and animals were collected from (B) Norra Flan and at some occasions the animals also were taken from (C) Södra Flan. The plants were collected from a shallow soft-bottom bay, Norra Flan (Fig. 1). All of the plants grew at a depth of approximately 1.2 m. The site was chosen because all of the concerned species were well represented. In the laboratory the plants were rinsed from epiphytes, eggs, roe and fauna. For each habitat 22 g wet weight was planted (7.33 g * 3 in the mix). The wet weight was obtained by spinning the plants in a salad spinner for about 1 minute and thereafter weighted (Delta range PB303; 2mg-310g, deviation=1mg). The plant biomass was decided by estimating the abundance of the plants in the field. Plants were kept for 3-5 days in sea-tempered, oxygenated water under ~80 PAR before being used in the experiment. In the experimental boxes plants were planted on an area of 5.25 dm2 (1.5 * 3.5 dm). The pair-wise comparisons of the four habitats resulted in 6 combinations. Hence, one replicate of all pair-wise combinations was constituted of 6 boxes. These were kept in a large tub (20.5 * 6 20.5* 6 dm) with circulating seawater, taken from a depth of 15 m, to cool the water in the containers. Two replicates were run at the same time and a total of 9 replicates were done over a 5 weeks period. The animals were collected from two sites, Norra Flan and an adjacent shallow bay, Södra Flan. The animals were collected from Fucus vesiculosus plants growing on the border between hard and soft-bottom.