Kristina Sundbäck

Consumers mediate the effects of experimental ocean acidification and warming on primary producers

It is well known that ocean acidification can have profound impacts on marine organisms. However, we know little about the direct and indirect effects of ocean acidification and also how these effects interact with other features of environmental change such as warming and declining consumer pressure. In this study, we tested whether the presence of consumers (invertebrate mesograzers) influenced the interactive effects of ocean acidification and warming on benthic microalgae in a seagrass community mesocosm experiment. Net effects of acidification and warming on benthic microalgal biomass and production, as assessed by analysis of variance, were relatively weak regardless of grazer presence. However, partitioning these net effects into direct and indirect effects using structural equation modeling revealed several strong relationships. In the absence of grazers, benthic microalgae were negatively and indirectly affected by sediment-associated microalgal grazers and macroalgal shading, but directly and positively affected by acidification and warming. Combining indirect and direct effects yielded no or weak net effects. In the presence of grazers, almost all direct and indirect climate effects were nonsignificant. Our analyses highlight that (i) indirect effects of climate change may be at least as strong as direct effects, (ii) grazers are crucial in mediating these effects, and (iii) effects of ocean acidification may be apparent only through indirect effects and in combination with other variables (e.g., warming). These findings highlight the importance of experimental designs and statistical analyses that allow us to separate and quantify the direct and indirect effects of multiple climate variables on natural communities.

The influence of benthic microalgae on the stability of a subtidal sediment

Abstract The relationship between benthic microalgae and the stabilisation of sediment was studied using a straight-channel laboratory flume tank. To establish different densities of microalgal biomass, sieved sediment collected from a shallow bay was kept in a laboratory flow-through aquarium either in darkness or with a 16 8 light/dark cycle. The relation between shear velocities causing sediment transport ( u ∗( crit ) ) and chlorophyll a content, content of colloidal polysaccharides, biomass and composition of microalgae in the top 5 mm of the sediment were studied. Microalgal samples were also collected using the cover-glass technique. Erosion tests were also made on undisturbed field samples. The investigation showed that the stabilizing effect of microalgae, previously reported for intertidal areas, may also apply for shallow-water sediments lacking regular tidal emersion. Initially, sieving decreased sediment stability by destroying consolidating structures. Within 5 days, however, values of u ∗( crit ) approached those of the undisturbed field samples (2.8 cm · s−1). The sediment that was exposed to light, became more stable against erosion (higher u ∗( crit ) ) than sediment kept in darkness. This difference is suggested to be caused by different densities of microalgae, as several sediment variables related to algae (content of chlorophyll a, polysaccharide content and algal biomass caught on cover glasses) were significantly different in light and dark treatments. The increased stability was mainly induced by the increased biovolume of motile diatoms within the size groups 20–40 μm and > 100 μm, as well as by filamentous cyanobacteria. Only the biovolume of algae trapped on cover glasses correlated significantly with critical shear velocity, while chlorophyll a, polysaccharides and biovolume in core samples showed positive, but non-significant correlations with critical shear velocity. Although the cover glasses harvested only a minor portion (5–10%) of the algal biovolume in the top 5 mm sediment, at least the motile diatom flora was better assessed by the cover-glass technique. Consequently, this appears to be a better indicator of sediment stability than total biomass or chlorophyll a content in the lop 5 mm sediment. Although there is a methodological uncertainty as to what components the variable “polysaccharides” include, polysaccharide content can be used to give at least a qualitative prediction of sediment stability. It is doubtful, however, whether any reasonable specific quantitative prediction about sediment stability can be made without an extensive knowledge about several factors, such as sediment structure, the quantity and the quality, as well as the physiological status of the organisms.

SHORT-TERM EFFECTS OF UVB RADIATION ON CHLOROPHYLL FLUORESCENCE, BIOMASS, PIGMENTS, AND CARBOHYDRATE FRACTIONS IN A BENTHIC DIATOM MAT

The effects on UVB radiation on a subtidal, cohesive‐sediment biofilm dominated by the diatom Gyrosigma balticum (Ehrenberg) Rabenhorst were investigated. Chlorophyll fluorescence parameters (Fv/Fm, φPSII), pigment concentrations, cell densities, and carbohydrate fractions were measured in four treatments (no UVBR, ambient UVBR, +7%, and +15% enhancement with UVBR). Enhanced UVBR was provided by a computer‐controlled system directly linked to natural diel UVBR levels. Increases in φPSII values in the UVBR‐enhanced treatments and a decrease in the steady‐state fluorescence yield (Fs) from the surface of the biofilms during the middle and latter part of daily exposure periods suggested that G. balticum responded to enhanced UVBR by migrating down into the sediment. Diatoms in the +15% UVBR treatment also had significantly higher concentrations of β‐carotene after 5 days of treatment. Although G. balticum responded to enhanced UVBR by migration and increased β‐carotene concentrations, significant reduction in maximum quantum yield of PSII (Fv/Fm) and in minimal fluorescence (Fo) and decreases in cell densities occurred after 5 days. Concentrations of different carbohydrate fractions (colloidal carbohydrate, glucan, exopolymers [EPS]) associated with diatom biomass and motility also decreased in the UVBR‐enhanced treatments. Short‐term responses (migration) to avoid UVBR appear insufficient to prevent longer‐term decreases in photosynthetic potential and biofilm carbohydrate concentration and biomass.

Experimental climate change weakens the insurance effect of biodiversity

Ecosystems are simultaneously affected by biodiversity loss and climate change, but we know little about how these factors interact. We predicted that climate warming and CO (2) -enrichment should strengthen trophic cascades by reducing the relative efficiency of predation-resistant herbivores, if herbivore consumption rate trades off with predation resistance. This weakens the insurance effect of herbivore diversity. We tested this prediction using experimental ocean warming and acidification in seagrass mesocosms. Meta-analyses of published experiments first indicated that consumption rate trades off with predation resistance. The experiment then showed that three common herbivores together controlled macroalgae and facilitated seagrass dominance, regardless of climate change. When the predation-vulnerable herbivore was excluded in normal conditions, the two resistant herbivores maintained top-down control. Under warming, however, increased algal growth outstripped control by herbivores and the system became algal-dominated. Consequently, climate change can reduce the relative efficiency of resistant herbivores and weaken the insurance effect of biodiversity.

Effect of Sediment Load on the Microbenthic Community of a Shallow-Water Sandy Sediment

Anthropogenic activities, such as construction work, dredging, and different kinds of recreation activities, can alter sediment loading in shallow coastal areas. The effect of increased load of fine sediment on the microbenthos (benthic microalgae, bacteria, and meiofauna) was studied in two experiments using undisturbed cores of a sandy sediment from a microtidal bay on the Swedish west coast. In each experiment, a total of 24 cores were incubated in an outdoor flow-through set-up. Twelve cores were treated with a 2.5-mm thick layer of autoclaved fine-grained, (silt) carbon-rich surface sediment. In the first experiment, estimates of the impact were based on measurements of chlorophylla, biomass of microalgae, bacteria, and meiofauna, and bacterial production. The main purpose of the second experiment was to study the effect on sediment oxygen profiles using microsensors. Within a week, after being covered by fine sediment, benthic microalgae (particularly diatoms) had migrated upward and the oxygen profiles were restored at the sediment surface by photosynthesis. However, the oxygen-producing layer became thinner and the algal composition changed. Bacterial biomass was restored to the same level as in the sandy sediment. Meiofauna also appeared to move upward and the meiofaunal composition was re-established. The results suggest that the microbenthic community of sandy sediment has an inherent capacity to recover after a moderate deposition of fine-particle sediment. Active upward migration of benthic diatoms appears to be a key mechanism for restoring the oxygenation of the sediment surface. The altered sediment type also implies changed species composition, and hence altered benthic trophic interactions, which may affect, for example, flatfish recruitment.

Amino acid uptake in natural microphytobenthic assemblages studied by microautoradiography

Grain density microautoradiography (MAR) was used to study uptake of dissolved amino acids (DFAA) in microalgal communities from a sandy bay on the west coast of Sweden. A mixture of fifteen 3H-labeled amino acids (final concentration 20–80 nmol l−1) was added to sediment samples collected from two depths (0.5 and 4 m), on five occasions representing different seasons. On all sampling occasions, the microflora was dominated by diatoms (> 85% of the total biomass). Cyanobacteria occurred in the summer and autumn, but never dominated the biomass. Between 5 and 48% of all counted algal cells showed uptake (1–44% of algal biomass). Uptake was recorded for all majors microalgal groups (diatoms, cyanobacteria and autotrophic flagellates). Uptake was more frequent on the shallow site and the highest proportion of cells showing uptake occurred in May. Although uptake was common among both motile and attached growth forms, on several occasions uptake frequencies were higher for attached cells. As the attached fraction is frequently burried out of the photic zone, and not able to rapidly migrate towards light, this implies that heterotrophic capacity should be valuable. Also at species level, some taxa showed higher uptake frequencies than others, for example populations of motile diatom species that displayed a seasonal behaviour (‘blooming species’), such as Nitzschia cf. dissipata and Cylindrotheca closterium. Other species showing frequent uptake were the coccoid colony-forming cyanobacterium Microcrocis sp., and the filamentous cyanobacterium Phormidium sp.. The results suggest that there is a high potential for DFAA uptake in the microphytobenthic community and that for some growth forms and species this could imply a competitive advantage.

Distribution of benthic diatoms in the littoral zone of the Gulf of Riga, the Baltic Sea

The composition (% relative abundance) of the benthic diatom community was studied in the littoral zone (0–5 m) of the NW and E coasts of the brackish, non-tidal Gulf of Riga, in the Baltic Sea. The samples were collected concomitantly with measurements of sediment primary productivity and chlorophyll a during three cruises, representing three seasons (autumn, spring, summer). At all depths, the living diatom community was dominated by epipsammic species, the most abundant taxa being Martyana atomus, a species of Fragilariaceae and Achnanthes delicatula. The community was characterized by a stable composition and a high species diversity throughout the study period (Shannon–Weaver, mean H′ = 4.4). Principal Component Analyses (PCA), together with Redundancy Analysis (RDA) based on 129 taxa, revealed both spatial (site and depth) and seasonal differences in the diatom communities. However, these differences were mainly caused by the varying proportions of taxa common in both sampling areas in all three seasons, rather than by actual major taxonomic changes in the species present in communities. The distribution of the typical epipsammic species along the depth gradient was not uni-directional. The dominance of attached life forms, and the absence of a well-developed epipelic community, reflects the physically dynamic substratum, particularly in the eastern part of the Gulf. As a result, primary productivity is kept rather low, decreasing rapidly with depth due to frequent turbidity of the water column.

Nitrogen dynamics in nontidal littoral sediments: Role of microphytobenthos and denitrification

Previous measurements from cool microtidal temperate areas suggest that microphytobenthic incorporation of nitrogen (N) exceeds N removal by denitrification in illuminated shallow-water sediments. The present study investigates if this is true also for fully nontidal sediments in the Baltic Sea., Sediment-water fluxes of inorganic (DIN) and, organic nitrogen (DON) and oxygen, as well as denitrification, were measured in early autumn and spring, in light and dark, at four sites representing different sediment types. All sediments were autotrophic during the daytime both in the autumn and spring. On a 24-h time scale, they were autotrophic in the spring and heterotrophic in early autumn. Sediments funcitoned as sources of DIN and DON during the autumn and sinks during the spring, with DON fluxes dominating or being as important as DIN fluxes. Microphytobenthos (MPB) activity controlled fluxes of both DIN and DON. Significant differences between sites were found, although sediment type (sand or silt) had no consistent effect on the magnitude of MPB production or nutrient fluxes. The clearest effect related to sediment type was found for denitrification, although only in the autumn, with higher rates in silty sediments. Estimated N assimilation by MPB, based on both net primary production (0.7–6.5 mmol N m−2 d−1) and on 80% of gross primary production (1.9–9.4 mmol N m−2 d−1) far exceeded measured rates of denitrification (0.01–0.16 mmol N m−2 d−1). A theoretical calculation showed that MPB may incorporate between 40% and 100% of the remineralized N, while denitrification removes, <5%. MPB assimilation of N appears to be a far more important N consuming process than denitrification in these nontidal, shallow-water sediments.

An upright life-form of an epipelic motile diatom: on the behaviour of Gyrosigma balticum

Previously undescribed behaviour of the large (≈ 400 μm) motile diatom Gyrosigma balticum was observed in intact sediment cores from a non-tidal mudflat. Instead of lying flat against the sediment surface, the individual cells were found orientated with their long axes perpendicular to the sediment surface. Staining with toluidine blue indicated that the mechanism behind the erect position was a short (100–200 μm) mucilage tube produced at the lower end of the cell. It is suggested that the upright life-form may be common among epipelic diatoms. A diurnal/vertical rhythm into and out of the sediment was also observed. The peak of the oxygen profile in the Gyrosigma mat occurred 250–300 μm above the sediment surface, i.e. within the layer of upright cells.

Effect of the brown shrimp Crangon crangon L. on endobenthic macrofauna, meiofauna and meiofaunal grazing rates

Abstract The effect of natural densities (50 and 100 ind·m −2 ) of juvenile ( Crangon crangon on abundance of endobenthic macrofauna and meiofauna and on meiofaunal grazing rates was investigated in two experiments using an outdoor flow-through system with sandy sediment. The experiments differed in duration (3 wk and 7.5 wk), and in time of the year (August – September and July – August). Macrofaunal biomass differed by a factor of 10 between the experiments, and was depressed by shrimp in both experiments. Neither total meiofaunal biomass, nor the biomass of the dominant taxon (nematodes), was significantly affected by the presence of Crangon in the shorter experiment. However, harpacticoid copepods and other meiofauna taxa (mainly ostracods, foraminiferans and juvenile bivalves) decreased in the presence of Crangon . In the longer experiment, no significant effect of Crangon on meiofauna was seen, and the biomass of most meiofaunal groups increased in all treatments. Meiofaunal grazing rates in microalgae and bacteria, measured with a dual-labelling method, using 14 C-bicarbonate and [methyl- 3 ]]-thymidine as tracers, were 0.7 to 4.7% of algal biomass per day, and 0.8 to 7.9% of bacterial biomass per day. Generally, grazing rates were lower in the presence than in the absence of Crangon . In terms of carbon ingested, microalgae constituted a more important food source than bacteria for all three meiofaunal groups (nematodes, harpacticoids and other meiofauna). No individual growth of Crangon was found in the high-density treatment in the longer experiment, suggesting that intraspecific competition occurred. The difference between the two experiments in the effect of Crangon on meiofauna is thought to be caused by the large difference in macrofaunal biomass between the experiments; the shrimp ate mainly juvenile macrofauna in the longer experiment. Overall, the effect of Crangon on the sediment system was weak, suggesting that other factors (physical and chemical) are more important than epibenthic predation in setting the overall limits for production in this sediment.