Erik Bonsdorff

Coastal eutrophication: Causes, consequences and perspectives in the Archipelago areas of the northern Baltic Sea

Coastal eutrophication has, since the early 1970s, become the foremost threat to the marine ecosystem ofthe Archipelago Sea (the Aland Islands and the SW Finnish archipelago) in the northern Baltic Sea. Nutrient levels (N, P) have risen significantly both in coastal areas and basin-wide, which has led to increased primary production (both pelagic and benthic), decreased transparency, increasing amounts of oxygen-consuming drift-algal mats at shallow and intermediate bottoms, and changes in zoobenthos and fish communities. Local nutrient input originates mainly from agriculture, riverine input, municipal wastewaters, aquaculture and airborne loading. Levels indicate an even distribution of nutrients from the inner areas to the open coast, reducing the natural diluting or filtering effects of the mosaic archipelago system. Future prospects for the archipelago and coastal ecosystem are poor unless local and regional measures to drastically reduce nutrient levels of the archipelago are undertaken. Even then, positive effects are unlikely to show immediately.

Community structure and spatial variation of benthic invertebrates associated with Zostera marina (L.) beds in the northern Baltic Sea

Abstract The distribution and bed structure of eelgrass ( Zostera marina L.), and its importance for associated faunal communities in the coastal areas of the northern Baltic Sea are poorly known. The spatial distribution of the fauna associated with Zostera was studied at five localities in SW Finland in 1993–1994. Zostera was common on all localities, but the beds varied in terms of area (1–5 m diameter), density (50–500 shoots/m 2 ) and blade length (20–110 cm). A total of about 40 species or taxa were recorded. The zoobenthic infauna showed significant spatial differences, and total abundance and species diversity were significantly higher in the Zostera beds than in adjacent bare sand. The total abundance in Zostera ranged from 25 000 to 50 000 ind/m 2 and in sand from 2500 to 15 000 ind/m 2 The mean number of species in Zostera ranged from 5.9 to 8.8 spp ( H ′ = 1.76–2.54) and in sand from 2.2 to 5.5 spp ( H ′ = 1.67–2.31). The epifauna in Zostera was numerically dominated by grazing gastropods (Hydrobiidae) and copepods. The epifauna is an important community component, which contributes to the total diversity of the Zostera assemblage. These systems are among the most species-rich components of the shallow soft-bottom ecosystems in the northern Baltic Sea. The mechanisms structuring both the Zostera and the ambient sand-bottom habitats are presented.

Drifting algal mats as an alternative habitat for benthic invertebrates:: Species specific responses to a transient resource

Patchy occurrences of benthic drift algae (i.e. loose lying macroalgal mats) may increase habitat complexity on normally bare soft bottoms, but at the same time, extensive amounts of drifting algal mats are known to stress the benthic fauna. This paper presents results of the first detailed study of the fauna associated with drift algal mats in the northern Baltic Sea. In order to assess the importance of drifting algae as an alternative habitat for benthic fauna, benthic drift algal mats were sampled on shallow (2-9 m) sandy soft bottoms in the outer archipelago of the Åland Islands (Finland). Species composition, abundance and biomass of the macrofauna associated with algal mats were recorded. The results show that drifting algae at times can harbour very high abundances of invertebrates (up to 1116 individuals/g algal dryweight), surpassing invertebrate densities recorded in seagrass communities. The algal fauna varied between sites and over time, and factors such as ambient benthic fauna, exposure to wind-wave disturbance, depth, and algal coverage and condition influenced the invertebrate community composition of the algal mats. Abundance increased while individual biomass of the animals decreased over time (summer season; July-October). A series of laboratory experiments were conducted in order to test the ability of a few important benthic species to move up into, and survive in a drifting algal mat. Macoma balthica, Hydrobia spp., Nereis diversicolor and Bathyporeia pilosa were used in the experiments, and significant differences in their survival and mobility within drifting algae were recorded. This study shows that benthic species differ significantly in their ability to utilise the algal mats, with mainly opportunistic and mobile taxa such as Hydrobia spp., Chironomidae and Ostracoda benefiting from the algae, whereas infaunal species such as M. balthica and B. pilosa are negatively affected. The occurrence of eutrophication induced drifting macroalgal mats has increased significantly during the last decade in the northern Baltic Sea. Hence, the importance of drifting algae as a stress factor and as an alternative habitat for benthic fauna increases.

Hypoxia Is Increasing in the Coastal Zone of the Baltic Sea

Hypoxia is a well-described phenomenon in the offshore waters of the Baltic Sea with both the spatial extent and intensity of hypoxia known to have increased due to anthropogenic eutrophication, however, an unknown amount of hypoxia is present in the coastal zone. Here we report on the widespread unprecedented occurrence of hypoxia across the coastal zone of the Baltic Sea. We have identified 115 sites that have experienced hypoxia during the period 1955–2009 increasing the global total to ca. 500 sites, with the Baltic Sea coastal zone containing over 20% of all known sites worldwide. Most sites experienced episodic hypoxia, which is a precursor to development of seasonal hypoxia. The Baltic Sea coastal zone displays an alarming trend with hypoxia steadily increasing with time since the 1950s effecting nutrient biogeochemical processes, ecosystem services, and coastal habitat.

Population responses of coastal zoobenthos to stress induced by drifting algal mats

In the large archipelago area of the northern Baltic Sea, increasing occurrences of drifting benthic macroalgae have been recorded in the subtidal zone. Their role as a structuring factor on the zoobenthic community has been altered from inducing occasional small-scale disturbances to inducing large-scale mortality of macrobenthic populations. A controlled field experiment was conducted on a sandy bottom in order to test for temporal responses of benthlc invertebrate populations to severe stress imposed by the algal mats. Algae corresponding to amounts recorded in the field were enclosed In netbags and attached to the bottom. Population abundance of the zoobenthic species under algae were compared w ~ t h control plots for 5 ivk with weekly sanipl~ng. Mass~ve die-offs of benthic populatlons were recorded in both the experiment and under natural occurrences of drift algal mats. The community dominants in abundance (mudsnails Hydrobia spp.) and biomass (bivalve Macoma balthica) exhibited strong population reductions after 9 d of algal stress. Within 5 wk, populat~on crashes were recorded for the sedentary po1ychaett.s Manayunkia aestuanna and Pygosplo elegans, ivh~le populations of the errant polychaete Nereis diverslcolor and tubificid oligochaetes remained stable under the algal mats. Initial short-term recovery after the algae were removed was rapid and do~nlnated by Hydrobia spp. The strong negative effects on key species such as M. balthica may have severe effects on entire food-web dynamics in Baltic coastal ecosystems. KEY MIORDSMacrofauna . Drifting macroalgae Disturbance . Baltic Sea . Macoma balthica . Hydrobia spp

Baltic Sea eutrophication: area-specific ecological consequences

Eutrophication of coastal waters is a global phenomenon. the amounts of nutrients in the brackish water of the Baltic Sea have increased several times during the last century, with severe ecological effects on the biota. With the increasing environmental problems caused by nutrient over-enrichment, public awareness to the problem has also risen. The Baltic Sea cannot be regarded as a uniform water mass, and area-specific ecological responses can be described. Changes in and detection of eutrophication-related parameters are discussed in relation to a generalized conceptual eutrophication model for the Baltic Sea. The cascading trophic and ecosystem-responses to eutrophication in 9 different sub-regions of the Baltic Sea are illustrated and discussed. The results clearly show the need not only for a common remedy for the Baltic Sea, but primarily show the importance of regional ecological assessment in relation to basin-wide eutrophication.

Drifting algae and zoobenthos — Effects on settling and community structure

Shallow (5 to 10 m) sandy bottoms in the Baltic Sea are important areas for zoobenthic production. The infaunal communities are generally governed by the hydrographical conditions are transport of the sediment through wind effects. With increasing eutrophication in the Baltic Sea, drifting mats of annual algae (Cladophora, Stictyosiphon, Polysiphonia, Rhodemela, Sphacelaria, Pilayella, Furcellaria, Ceramium, etc) have become increasingly common, adding to the structuring and regulating factors for the infauna. In 1990 and 91, a field-study (SCUBA diving; zoobenthos and algae sampling) was carried out in the Aland archipelogo, in thennorthern and their structuring effect on the zoobenthos. Algal biomass increased from 150 ± 19 g DW·m−2 in 1990 to 832±60 g DW·m−2 in 1991, having no effect on oxygen saturation in 1990, but showing signs of reduced oxygen saturation in 1991. Organic content of the sediment remained stable (0.60 to 0.74%) during the entire study period. The zoobenthic community showed significant responses to the drifting algae at population level and in terms of community structure (by 1991: significantly reduced species number; low similarity values (40 to 65%) between bare sand and under the algae). The main species affected were the dominating bivalve Macoma balthica, the polychaetes Pygospio elegans and Manayunkia aestuarina, and the amphipod Corophium volutator. The settlement of M. balthica spat was significantly reduced by the algae (>70% in 1990/91), and no individuals of the dominating polychaetes were recorded under the mat. C. volutator, however, benefited from the algae, and greatly increased in numbers. The results clearly demonstrate the types of physical effects drift-algae will have no sandy-bottom benthos, and show that significant changes in the communities over large areas can be expected with increasing eutrophication.

The Effect of Spatial and Temporal Heterogeneity on the Design and Analysis of Empirical Studies of Scale-Dependent Systems

Processes interacting across scales of space and time influence emergent patterns in ecological systems, but to obtain strong inference and empirical generalities, ecologists need to balance reality with the practicalities of design and analyses. This article discusses heterogeneity, scaling, and design analysis problems and offers potential solutions to improve empirically based research. In particular, we recommend bridging the dichotomy between correlative and manipulative studies by nesting manipulative studies within a correlative framework. We suggest that building on variation, by designing studies to detect variability, rather than fighting it often leads to an increase in generality. We also emphasize the importance of natural history information for determining likely scales of spatial and temporal heterogeneity and the probable occurrence of feedback loops, indirect effects, and interacting processes. Finally, we integrate these concepts and suggest planned iterations between multiscale studies to build up natural history information and test the strength of relationships across space and time. This offers a way forward in terms of heuristically developing models and determining ecological generalities.

Food and feeding habits of juvenile flounder Platichthys flesus (L.), abd turbot Scophthalmus maximus L. in the åland archipelago, northern Baltic Sea

Abstract The food choice of juvenile flounder ( Platichthys flesus ) and turbot ( Scophthalmus maximus ) was studied in the northern Baltic Sea during the years 1988, 1989, 1994 and 1995. The diet included organisms from 30 species/taxa in flounder (n = 306) and 10 species/taxa in turbot (n = 41). Flounder ⩽ 45 mm mainly consumed meiofauna (dominating taxon: Harpacticoida, Copepoda) and larger fish (46–101 mm) consumed macrofauna (dominating taxa: Oligochaeta, Amphipoda and Chironomidae). In terms of biomass, macrofauna dominated for all sizes of flounders, and meiofauna was important only for the smallest fish. A strong seasonal variation could be detected in the diet. In spring, macrofauna dominated for all size classes of fish (only fish > 30 mm were caught in spring), while in summer and autumn meiofauna dominated the diets for fish ⩽ 45 mm in size. Juvenile turbot (22–88 mm) consumed macrofauna and small fish. Turbot ⩽ 30 mm consumed mainly amphipods, while > 30 mm turbot consumed mysid shrimps, amphipods and fish. The ontogenetic shift from meio- to macrofauna-sized prey in flounders occurs at a larger fish size in the northern Baltic Sea than reported in other areas, possibly depending on the increased relative importance of meiofauna in the northern Baltic. The seasonal variation in the diet could be due to seasonally changing abundances in the zoobenthos, or for the small fish (1-group, spring), to switching from meio- to macrofauna in order to optimize their energy gain. The 0-group flounders consumed meiofauna for a long period, possibly due to a learning-process or simply due to easy availability of meiofauna. Turbot has a much larger mouth gap than flounders, thus allowing them to consume macrofauna from the beginning of their benthic life.

Fish predation and habitat complexity: are complexity thresholds real?

Empirical data have suggested the existence of a substratum complexity threshold below which predation rates are not affected by variations in habitat complexity. Complexity thresholds might either be real, resulting from switching of predator behavior, or artifacts of experimental designs, resulting from too few experimental treatments broadly separated in habitat complexity or from inadequate statistical power of the experiment. Experimental work with perch Percajluviatilus L. and stickleback Pungitius pungitius L. prey in simulated reed beds utilized extensive replication (20) of 11 habitat complexity treatments to examine this question. Predation rates were a linear function of habitat complexity level. This linear function accurately predicted predation rates for higher complexity levels than initially used, suggesting that no complexity threshold occurred for perch. Although these results argue against the existence of habitat complexity thresholds, additional tests with other predators will be necessary to answer this question.