Claes Nilsson

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.

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.

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.

Does the influence of the epibenthic predator crangon crangon L. (brown shrimp) extend to sediment microalgae and bacteria

Abstract The effect of juvenile Crangon crangon L. (brown shrimp) on the microbial part of the food web of marine shallow-water sandy sediment was investigated in two experiments using an outdoor flow-through system. Biomass, composition and productivity of microalgae and bacteria, POC and PON content in the sediment, as well as nutrient and oxygen fluxes, were measured in the absence and presence of two natural densities (50 and 100 ind·m −2 ) of juvenile Crangon . Stimulating effects of the presence of juvenile Crangon on both microalgae and bacteria were observed after three weeks. However, the number of statistically significant effects was low, despite the conspicuous change in sediment characteristics caused by Crangon activity. To explain the possible trophic effect of Crangon via meiofaunal grazing, the results were compared with meiofaunal grazing rates (dual labelling by 14 C-bicarbonate and 3 H-thymidine) and changes in the meiofaunal and macrofaunal community. The reasons for the low number of significant trophic effects of Crangon on algae and bacteria were that the overall meiofaunal grazing pressure on microbiota was low, and that only part of the meiofauna (mainly harpacticoid copepods) was significantly affected by the predator. Meiofauna thus appeared to be a weak mediating link in the sandy sediment system investigated. No effect of Crangon on oxygen fluxes, or POC and PON content in the sediment was observed. Neither were the nutrient concentrations in the overlying water affected, but increased nutrient levels in the pore water were observed. According to theoretical calculations, excretion from the shrimp may imply a considerable nutrient addition which can be rapidly re-utilized by the microbes directly at the sediment/water interface. Bioturbation by Crangon appeared to counteract flaking of the microalgal mat.

Growth and nutrient uptake studied in sand-agar microphytobenthic communities

Abstract A sand-agar microalgal community was used in an outdoor flow-through experiment to study the response of marine microphytobenthos to changed nutrient levels mediated by the activity of a predator (brown shrimp Crangon crangon L.) and by addition of inorganic nitrogen (N) and phosphorus (P) to the overlying water. A microalgal assemblage was separated from ambient sandy sediment and allowed to settle onto thin sand-agar plates in order to minimize the interactive effects of other biogeochemical processes than algal uptake on nutrient flux. The response of the algal community was assessed by measuring changes in composition, biomass, primary productivity, and nutrient uptake rates. A natural dense population of juvenile Crangon did neither affect the inorganic nutrient concentration in the overlying water nor the sand-agar microalgal community. The NP addition stimulated algal growth and productivity. The proportion of flagellates increased with NP addition, although a clear shift in the dominance of the major algal groups was not found in the course of the 3-wk experiment. The initial microalgal community on the sand-agar substratum did initially imitate that of shallow-water sediments in the respect that the major taxonomical and morphological groups were the same. During the experiment the composition of diatoms was shifted to a dominance of larger motile cells at the expense of small epipsammic forms, probably both as a result of the nature of the substratum and the lack of abrasion by moving sand grains. The response of the algal community to changed nutrient status had an acceptably high degree of similarity to that of the sediment. The fact that the quantitative response of the algal community was faster and more pronounced in the absence of sediment is an advantage when the aim is to simplify a system for laboratory experiments. The nutrient uptake measured was shown to reflect algal uptake with low interactive effects of biogeochemical processes present in natural sediment. Thus, a thin sand-agar substratum appears to be a useful tool in short-term experiments where it is desirable to simplify the complicated sediment community to specifically study nutrient uptake as well as response of the microphytobenthos to environmental changes.