Johanna Mattila

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.

The relative importance of food and shelter for seagrass-associated invertebrates: a latitudinal comparison of habitat choice by isopod grazers

Abstract The generality of mechanisms affecting habitat choice and grazing in seagrass meadows was evaluated in a latitudinal comparison of seagrass grazers from the temperate (60°N) Baltic Sea and the subtropical (30°N) Gulf of Mexico. Using similar habitat choice experiment set-ups in Finland and the USA, the role of food type, habitat complexity and predation hazard on habitat choice of the isopods Idotea baltica (Pallas) and Erichsonella attenuata Harger were tested. When shelter was provided by both living and artificial seagrass, epiphytic food resources on artificial vegetation were clearly preferred by both species, although Idotea was attracted to epiphyte-free seagrass when no alternative food was present. When choosing between food and shelter, both species preferred epiphytic food over shelter. However, under predation hazard of fish, Erichsonella clearly switched to the habitat offering shelter, while the presence of a predatory fish produced no preference for shelter by Idotea. Food type may be considered as an universal mechanism that partly determines the presence of grazers in seagrass habitats and is, in the absence of a predator, more important than shelter. Predation risk affected the behaviour of the grazers, but the response varied between species possibly due to varying importance of fish predation in the areas studied.

Spatial and diurnal distribution of invertebrate and fish fauna of a Zostera marina bed and nearby unvegetated sediments in Damariscotta River, Maine (USA)

Abstract Fish, epibenthos and macroinfauna were collected in a Zostera marina bed and nearby unvegetated sediments in the estuary of the Damariscotta River, on the mid-coast of Maine. Samples of epibenthic fauna and fish were collected at low tides both during day and night, and samples of infauna at low tides during the day. The mean density of Zostera shoots in the study area was 335 m −2 . Abundance and species number of fish were greater at night than during the day and greater in eelgrass beds ( Z. marina ) than in unvegetated habitats. Daytime fish collections were dominated by Atlantic silversides ( Medinia medinia ), while juvenile winter flounder ( Pseudopleuronectes americanus ) dominated night collections. Also Zostera -associated epifaunal abundances and number of species were significantly higher at night than during the day. Mysis stenolepis , Idotea balthica and Littorina obtusata were dominant species in the epifauna samples. Of the total of 37 invertebrate species encountered, only five occurred both in the infaunal and epifaunal samples. Nineteen different taxa were collected from the benthic core samples. The most abundant invertebrate infaunal taxa were sipunculids, the polychaete Nereis virens , and oligochaetes. Infaunal invertebrate abundances and species diversity were significantly higher in eelgrass beds than in unvegetated sediments. The abundance and number of species of benthic invertebrates were also positively correlated to seagrass biomass. Community diversity values ( H ′) were relatively low but fit well in the general pattern of decreasing diversity towards northern latitudes.

Seasonal and inter-annual variation in occurrence and biomass of rooted macrophytes and drift algae in shallow bays

Submerged rooted macrophytes and drift algae were studied in shallow (0-1 m) brackish soft-bottom bays in the Aland Islands, N Baltic Sea, in 1997-2000. The study was performed by aerial photography and ground-truth sampling and the compatibility of the methods was evaluated. The study provided quantitative results on seasonal and inter-annual variation in growth, distribution and biomass of submerged macrophytes and drift algae. On an average, 18 submerged macrophyte species occurred in the studied bays. The most common species, by weight and occur- rence, were Chara aspera, Cladophora glomerata, Pilayella littoralis and Potamogeton pectinatus. Filamentous green algae constituted 45-70% of the biomass, charophytes 25-40% and vascular plants 3-18%. A seasonal pattern with a peak in biomass in July-August was found and the mean biomass was negatively correlated with exposure. There were statistically significant differences in coverage among years, and among levels of exposure. The coverage was highest when exposure was low. Both sheltered and exposed bays were influenced by drift algae (30 and 60% occurrence in July-August) and there was a positive correlation between exposure and occurrence of algal accumulations. At exposed sites, most of the algae had drifted in from other areas, while at sheltered ones they were mainly of local origin. Data obtained by aerial photography and ground-truth sampling showed a high concordance, but aerial photography gave a 9% higher estimate than the ground-truth samples. The results can be applied in planning of monitoring and management strategies for shallow soft-bottom areas under potential threat of drift algae. 2003 Elsevier Science B.V. All rights reserved.

The effect of habitat complexity on predation efficiency of perch Perca fluviatilis L. and ruffe Gymnocephalus cernuus (L.)

Abstract Aquarium experiments were conducted to test the hypothesis that a slight increase in habitat complexity significantly increases the survival of the amphipods Corophium volutator Pallas and the isopod Asellus aquaticus (L.) when preyed upon by perch Perca fluviatilis L. and ruffe Gymnocephalus cernuus (L.), respectively. Artificial reed, stones, and natural plants were tested in different combinations. A slight increase in habitat complexity (e.g., small amount of stones) did not increase Corophium survival. However, artificial reeds in different forms, and natural plants, did increase the survival of both Asellus and Corophium . Survival increased with increasing habitat complexity. At low complexity levels the survival of the prey seemed to be higher when the elements used to provide complexity were in bigger integrated patches. Tall, shading elements like reeds or aquatic plants gave the best shelter against fish that feed visually.

Changes in zoobenthic community structure after pollution abatement from fish farms in the Archipelago Sea (N. Baltic Sea)

Long-term changes in sediment macrofauna communities at two sites affected by fish farming in the Archipelago Sea, south-west Finland have been investigated. Sampling stations in the Särkänsalmi Strait and Kaukolanlahti Bay, previously investigated 1982-1991, were revisited in 1994, 1995 and 1998 to detect signs of recovery following a decrease in organic load since 1990 and 1991, respectively. The results indicate a partial recovery in Särkänsalmi during post-pollution years, whereas no improvement has taken place in Kaukolanlahti. The improvement in Särkänsalmi is shown by a significant increase in the number of species and total abundance, and by the community structures becoming more similar over time. On the other hand, a significantly decreased number of species, abundance and biomass values over time as well as the occurrence of defaunated anoxic sediments, are clear signs of continued deterioration in Kaukolanlahti. Differences in the recovery potential of the two water areas are interpreted as consequences of topography and water exchange patterns causing differences in oxygen saturation.

Predation by juvenile flounder (Platichthys flesus L.): a test of prey vulnerability, predator preference, switching behaviour and functional response

Abstract Predation by juvenile 1+flounders, Platichthys flesus , (mean total length 8.0 cm±0.6 cm SD) on juvenile Macoma balthica (mean length 1.94 mm±0.40 mm SD) and Bathyporeia pilosa (mean length 4.11 mm±0.47 mm SD) was studied in a series of aquarium experiments. Prey vulnerability and consumption, as well as possible switching behaviour by the predator, were studied with varying total and relative prey densities. The functional response of the predator was also determined from the experimental results. The consumption and vulnerability of both prey species were equal when the prey species were presented separately to the predators (50 ind/aq). In the comparable two-prey-species experiment, where equal proportions of the prey species were presented to the predators, total prey consumption was about the same as in the single-prey-species experiment. Prey consumption during the night was clearly lower than prey consumption during the day with the same prey density. In all experiments, juvenile flounder showed a slight preference for Macoma before Bathyporeia independently of the relative or absolute prey density. No switching behaviour was observed. The consumption of both Macoma and Bathyporeia increased significantly with increasing total prey density. The functional response of flounder was in both cases of the type III. The best fit of the data was achieved with a continuous model of the type-III response.

The effects of vegetation density on the relative growth rates of juvenile pinfish, Lagodon rhomboides (Linneaus), in Big Lagoon, Florida

Abstract The efficiency of visual predators may often be affected by complexity of habitat. For example, an increase in complexity of habitat may lead to an increase in search and/or pursuit times, while decreasing habitat complexity may reduce search and/or pursuit times and result in elevated feeding success. If true, it might be expected that predator rates of growth would be greater in small complexity habitats and decline with increasing complexity of habitat. In shallow turtlegrass ( Thalassia testudinum ) meadows, we used field enclosures to evaluate the effect of increasing turtlegrass density (small=0–133 shoots/m 2 , intermediate=177–267 shoots/m 2 , and large=>267 shoots/m 2 ) on the growth rates of juvenile pinfish, Lagodon rhomboides (Linneaus). Experiments were carried out five times during the growing season, with individual experiments lasting from 21 to 28 days. Benthic core samples were taken to examine differences among treatments and possible caging effects on plant surface area, faunal abundance and epiphyte coverage. We found a consistent pattern of decreasing growth with increasing density of vegetation, although due to low power, ANOVA showed no significant differences among treatments. However, the probability of obtaining the same treatment rankings we observed by chance was very low ( P =0.003). The effects of vegetation density on annual growth rates were also examined, and we estimated a 20% decrease in growth rates between small and large vegetation density treatments. These results suggest that vegetation density can have an impact on growth rates that is biologically significant.

Predation by juvenile Platichthys flesus (L.) on shelled prey species in a bare sand and a drift algae habitat

Due to increasing eutrophication of the coastal Baltic waters, drifting algae are a common phenomenon. Drifting algal mats accumulate on shallow sandy bottoms in late summer and autumn, and affect the ambient fauna. Juvenile flounder, Platichthys flesus, utilize these habitats during their first few years. They feed on benthic meio- and macrofauna; part of their diet consists of shelled species, such as Ostracods, and juvenile Hydrobia spp. and Macoma balthica. Earlier studies have shown that up to 75% of ostracods and 92% of hydrobiids survive the gut passage of juvenile flounder, while all M. balthica are digested by the fish. We conducted laboratory experiments to study how the shelled prey responded to a drift algal mat, and the predation efficiency of juvenile P. flesus on these prey species on bare sand and with drifting algae (50% coverage). Hydrobia spp. utilized the drift algae as a habitat and, after 1 h, 50% had moved into the algae; ostracods and M. balthica were more stationary and, after 96 h, only 23 and 12%, respectively, were found in the algae. For the predation efficiency of P. flesus, a two-way ANOVA with habitat (algae, bare sand) and predation (fish, no fish) as factors revealed that both algae and predation affected negatively the survival of all three prey species. The algae, thus, affected the predation efficiency of juvenile P. flesus and the consumption of prey was much reduced in the algal treatments compared to the bare sand. This was due probably to increased habitat complexity and the ability of prey, especially hydrobiids, to use the algal mat as a refuge. Altered habitat structure due to drift algae, together with the resultant changes in habitat (refuge) value for different prey species, may profoundly change the structure of benthic communities.

Periphyton growth as an indicator of eutrophication; an experimental approach

Growth of periphyton was studied in the Archipelago Sea (Finland) during summer 1994 as a part of the annual monitoring control of fish farms. Experimental growth plates (Whatman GF/C filters) were incubated (two weeks) at 213 sites in different parts of the Archipelago Sea and in the coastal area north of the Archipelago Sea. At each site incubations were repeated two or three times during the summer (July 4- -September 1). The growth of periphyton was measured as the amount of chlorophyll a (mg m-2) extracted from the incubation plates.The growth varied significantly among different parts of the study area. The strongest growth was observed in the inner archipelago and in areas with high fish production and relatively slow water exchange. In the outer archipelago, rapid water exchange ensured good mixing of nutrients from fish farms and other sources into relatively large water volumes. The local impacts of nutrient loading thus remained too low to be detected by measuring nutrient concentrations or periphyton growth.Periphyton growth was positively correlated with concentrations of total nitrogen, total phosphorus and chlorophyll a in the productivity layer (approximately 2 × Secchi depth). A significant inverse correlation was also shown between periphyton growth and Secchi depth.Archipelago areas with different levels of eutrophication could satisfactorily be distinguished in this study. The results were consequent with previous classifications of the eutrophication levels in the Archipelago Sea. Periphyton studies thus are a useful addition to conventional monitoring programs.