Britas Klemens Eriksson

Declines in predatory fish promote bloom-forming macroalgae

In the Baltic Sea, increased dominance of ephemeral and bloom-forming algae is presently attributed to increased nutrient loads. Simultaneously, coastal predatory fish are in strong decline. Using field data from nine areas covering a 700-km coastline, we examined whether formation of macroalgal blooms could be linked to the composition of the fish community. We then tested whether predator or nutrient availability could explain the field patterns in two small-scale field experiments, by comparing joint effects on algal net production from nutrient enrichment with agricultural fertilizer and exclusion of larger predatory fish with cages. We also manipulated the presence of invertebrate grazers. The abundance of piscivorous fish had a strong negative correlation with the large-scale distribution of bloom-forming macroalgae. Areas with depleted top-predator communities displayed massive increases in their prey, small-bodied fish, and high covers of ephemeral algae. Combining the results from the two experiments showed that excluding larger piscivorous fish: (1) increased the abundance of small-bodied predatory fish; (2) changed the size distribution of the dominating grazers, decreasing the smaller gastropod scrapers; and (3) increased the net production of ephemeral macroalgae. Effects of removing top predators and nutrient enrichment were similar and additive, together increasing the abundance of ephemeral algae many times. Predator effects depended on invertebrate grazers; in the absence of invertebrates there were no significant effects of predator exclusion on algal production. Our results provide strong support for regional declines of larger predatory fish in the Baltic Sea promoting algal production by decreasing invertebrate grazer control. This highlights the importance of trophic interactions for ecosystem responses to eutrophication. The view emerges that to achieve management goals for water quality we need to consider the interplay between top-down and bottom-up processes in future ecosystem management of marine resources.

Long-term changes in the macroalgal vegetation of the inner Gullmar Fjord, Swedish Skagerrak coast

We examined long‐term changes in the macroalgal vegetation at Stora Bornö Island in the inner Gullmar Fjord on the Swedish Skagerrak coast. This was made possible by access to a 1941 diving investigation. The same sites were reinvestigated in 1998. Community composition and depth distributions of species were compared and changes were analyzed with focus on functional groups (size, thallus shape, and life‐history traits). We discovered a significant decrease in the depth extension of macroalgal species and a dramatic decline of species richness in the lower littoral (below 16 m of depth) compared with 57 years earlier. Ordination analysis revealed that there was a significant difference in the community composition between the two study periods. In general, small (<10 cm), thin, filamentous, and aseasonal ephemerals increased in relative abundance, whereas larger (>10 cm), coarsely branched, and perennial algae decreased. Calibrations of individual species to local sediment cover, using canonical correspondence analysis, indicated that part of the change in species composition was related to sediment load. Furthermore, large‐scale climate differences (NAO Winter Index) between the study periods indicated a higher impact of Baltic Sea and Kattegat water in the nutrient dynamics of the fjord in the 1998 study. We concluded that the observed long‐term changes in the macroalgal community at Stora Bornö Island were consistent with an increased nutrient availability.

Long-term changes in the sublittoral zonation of brown algae in the southern Bothnian Sea

The algal vegetation of five diving profiles situated at exposed sites of the outer Oregrund archipelago, southern Bothnian Sea, Sweden, was investigated by SCUBA diving in 1996. The vegetation of the same profiles had been described earlier in 1943-4 and 1984. In the present study, special attention was paid to the occurrence of the structurally important belt-forming brown algae Fucus vesiculosus and Sphacelaria arctica in comparison with the situation 52-53 and 12 years earlier. Prominent differences between the 1940s and 1996 were found. The weighted average depth of the F. vesiculosus belt in 1996 was about 1.7 m shallower than in the 1940s, and the lower distribution limit of this species was about 2.5 m shallower. At its lower end the Fucus belt was replaced by epilithic red algae, mainly Furcellaria lumbricalis, Rhodomela confervoides and Polysiphonia fucoides. These results were similar to those of the 1984 study and show neither an improvement nor a deterioration of the belt during the past 12 years. In four of the five diving profiles, a conspicuous belt of S. arctica occurred in the lower sublittoral, similar to that observed in the 1940s, but in one of the five profiles the S. arctica belt was heavily impoverished. These results contrast with the 1984 study which reported that S. arctica had practically disappeared from all profiles. The epiphytic cover of annual filamentous algae, dominated by Pilayella littoralis and Ceramium gobii, was higher in 1996 than in 1943-4. The results of this comparative study are discussed in relation to the general eutrophication of the Baltic Sea.

Biodiversity mediates top–down control in eelgrass ecosystems: a global comparative‐experimental approach

Nutrient pollution and reduced grazing each can stimulate algal blooms as shown by numerous experiments. But because experiments rarely incorporate natural variation in environmental factors and biodiversity, conditions determining the relative strength of bottom-up and top-down forcing remain unresolved. We factorially added nutrients and reduced grazing at 15 sites across the range of the marine foundation species eelgrass (Zostera marina) to quantify how top-down and bottom-up control interact with natural gradients in biodiversity and environmental forcing. Experiments confirmed modest top-down control of algae, whereas fertilisation had no general effect. Unexpectedly, grazer and algal biomass were better predicted by cross-site variation in grazer and eelgrass diversity than by global environmental gradients. Moreover, these large-scale patterns corresponded strikingly with prior small-scale experiments. Our results link global and local evidence that biodiversity and top-down control strongly influence functioning of threatened seagrass ecosystems, and suggest that biodiversity is comparably important to global change stressors.

Effects of Altered Offshore Food Webs on Coastal Ecosystems Emphasize the Need for Cross-Ecosystem Management

By mainly targeting larger predatory fish, commercial fisheries have indirectly promoted rapid increases in densities of their prey; smaller predatory fish like sprat, stickleback and gobies. This process, known as mesopredator release, has effectively transformed many marine offshore basins into mesopredator-dominated ecosystems. In this article, we discuss recent indications of trophic cascades on the Atlantic and Baltic coasts of Sweden, where increased abundances of mesopredatory fish are linked to increased nearshore production and biomass of ephemeral algae. Based on synthesis of monitoring data, we suggest that offshore exploitation of larger predatory fish has contributed to the increase in mesopredator fish also along the coasts, with indirect negative effects on important benthic habitats and coastal water quality. The results emphasize the need to rebuild offshore and coastal populations of larger predatory fish to levels where they regain their control over lower trophic levels and important links between offshore and coastal systems are restored.

Sedimentation reduces recruitment success of Fucus vesiculosus (Phaeophyceae) in the Baltic Sea

During the last few decades, Fucus vesiculosus has decreased throughout the Baltic Sea, where it is the dominant canopyforming macroalga of the sublittoral zone. Concurrently, the organic sedimentation originating from primary production has increased substantially due to a large-scale eutrophication. We conducted a field experiment to investigate the effect of sedimentation on the recruitment success of F. vesiculosus. We manipulated the depositional environment during the reproductive period of Fucus by regularly removing accumulated sediment on artificial substrate in the lower part of the depth distribution of the Fucus vegetation. The natural sediment load significantly reduced the density of F. vesiculosus juveniles on the artificial substrate, compared with the sediment removal treatment. It was mainly juveniles smaller than 5 mm that were affected. Our results also showed that F. vesiculosus has the potential to recruit below the lower depth limit of adult Fucus plants if clean surfaces are present. Although it has frequently been suggested that small-scale variation in the sediment load has a significant influence on the establishment of F. vesiculosus in field conditions, this paper presents the first experimental evidence for this. We conclude that the increased organic sedimentation in the Baltic Sea may have a limiting effect on the abundance and distribution of F. vesiculosus.

BIOTIC HABITAT COMPLEXITY CONTROLS SPECIES DIVERSITY AND NUTRIENT EFFECTS ON NET BIOMASS PRODUCTION

Canopy-forming plants and algae commonly contribute to spatial variation in habitat complexity for associated organisms and thereby create a biotic patchiness of communities. In this study, we tested for interaction effects between biotic habitat complexity and resource availability on net biomass production and species diversity of understory macroalgae by factorial field manipulations of light, nutrients, and algal canopy cover in a subtidal rocky-shore community. Presence of algal canopy cover and/or artificial shadings limited net biomass production and facilitated species diversity. Artificial shadings reduced light to levels similar to those under canopy cover, and net biomass production was significantly and positively correlated to light availability. Considering the comparable and dependent experimental effects from shadings and canopy cover, the results strongly suggest that canopy cover controlled net biomass production and species diversity by limiting light and thereby limiting resource availability for community production. Canopy cover also controlled experimental nutrient effects by preventing a significant increase in net biomass production from nutrient enrichment recorded in ambient light (no shading). Changes in species diversity were mediated by changes in species dominance patterns and species evenness, where canopy cover and shadings facilitated slow-growing crust-forming species and suppressed spatial dominance by Fucus vesiculosus, which was the main contributor to net production of algal biomass. The demonstrated impacts of biotic habitat complexity on biomass production and local diversity contribute significantly to understanding the importance of functionally important species and biodiversity for ecosystem processes. In particular, this study demonstrates how loss of a dominant species and decreased habitat complexity change the response of the remaining assembly to resource loading. This is of potential significance for marine conservation since resource loading often promotes low habitat complexity and canopy species are among the first groups lost in degraded aquatic systems.

Major changes in the ecology of the Wadden Sea: Human impacts, ecosystem engineering and sediment dynamics

Shallow soft-sediment systems are mostly dominated by species that, by strongly affecting sediment dynamics, modify their local environment. Such ecosystem engineering species can have either sediment-stabilizing or sediment-destabilizing effects on tidal flats. They interplay with abiotic forcing conditions (wind, tide, nutrient inputs) in driving the community structure and generating spatial heterogeneity, determining the composition of different communities of associated species, and thereby affecting the channelling of energy through different compartments in the food web. This suggests that, depending on local species composition, tidal flats may have conspicuously different geomorphology and biological functions under similar external conditions. Here we use a historical reconstruction of benthic production in the Wadden Sea to construct a framework for the relationships between human impacts, ecosystem engineering and sediment dynamics. We propose that increased sediment disturbances by human exploitation interfere with biological controls of sediment dynamics, and thereby have shifted the dominant compartments of both primary and secondary production in the Wadden Sea, transforming the intertidal from an internally regulated and spatially heterogeneous, to an externally regulated and spatially homogenous system. This framework contributes to the general understanding of the interaction between biological and environmental control of ecosystem functioning, and suggests a general framework for predicting effects of human impacts on soft-bottom ecosystems.

Boating and navigation activities influence the recruitment of fish in a Baltic Sea archipelago area

Abstract We studied the effects of boating and navigation activities on the recruitment of coastal fish in the Stockholm archipelago in the NW Baltic proper. The impacts were quantified by sampling metamorphosed young-of-the-year (Y-O-Y) fish in inlets adjacent to i) routes for medium-sized passenger ferries; ii) berths (small marinas) with small boats; and iii) references. Species with high preference for vegetation were negatively influenced by boating and navigation activities and species with low preference positively influenced. Pike (Esox lucius) Y-O-Y were significantly more abundant in reference areas, while bleak (Alburnus alburnus) were more abundant in dredged marinas. No statistically significant patterns were identified for perch (Perca fluviatilis) although there was a trend of low abundance along ferry routes. Many species of nearshore fishes are dependent on submerged vegetation as spawning and larval substrate, structural refuge and feeding habitat. Our results suggest that the negative effects from boating and navigation activities on the coverage and height of vegetation, especially on species of Chara and Potamogeton spp., may contribute to changes in the Y-O-Y fish community.

Spatially extended habitat modification by intertidal reef-building bivalves has implications for consumer-resource interactions

Ecosystem engineers can strongly modify habitat structure and resource availability across space. In theory, this should alter the spatial distributions of trophically interacting species. In this article, we empirically investigated the importance of spatially extended habitat modification by reef-building bivalves in explaining the distribution of four avian predators and their benthic prey in the Wadden Sea—one of the world’s largest intertidal soft-sediment ecosystems. We applied Structural Equation Modeling to identify important direct and indirect interactions between the different components of the system. We found strong spatial gradients in sediment properties into the surrounding area of mixed blue mussel (Mytilus edulis) and Pacific oyster (Crassostrea gigas) reefs, indicating large-scale (100s of m) engineering effects. The benthic community was significantly affected by these gradients, with the abundance of several important invertebrate prey species increasing with sediment organic matter and decreasing with distance to the reefs. Distance from the reef, sediment properties, and benthic food abundance simultaneously explained significant parts of the distribution of oystercatchers (Haematopus ostralegus), Eurasian curlews (Numenius arquata), and bar-tailed godwits (Limosa lapponica). The distribution of black-headed gulls (Chroicocephalus ridibundus)—a versatile species with many diet options—appeared unaffected by the reefs. These results suggest that intertidal reef builders can affect consumer-resource dynamics far beyond their own boundaries, emphasizing their importance in intertidal soft-bottom ecosystems like the Wadden Sea.