Maria Kahlert

CONTROL OF MICROBENTHIC COMMUNITIES BY GRAZING AND NUTRIENT SUPPLY

In periphyton communities, autotrophic algae and prokaryotes live in close spatial proximity to heterotrophic components such as bacteria and micro- and meiofauna. In factorial field experiments, we manipulated grazer access and nutrient supply to periphyton communities and measured the effects on algal, ciliate, meiofaunal, and bacterial biomass. We tested whether grazing macrozoobenthos affects all periphytic components (generalist consumption), whether nutrient effects propagate through the community, and whether interactions between the different periphyton groups allow for indirect feedback mechanisms. The experiments were conducted during three different seasons in a meso-eutrophic lake in Sweden (Lake Erken) and at an adjacent coastal marine site (Vaddo) of similar productivity, but with contrasting grazer fauna. We found strong direct effects of nutrients and grazing on algae at both sites. Algal biomass increased in fertilized treatments and was significantly reduced when grazers were present. The algae clearly dominated the system quantitatively and were positively correlated to the biomass of ciliates and meiofauna. The effects of grazing and nutrients were more complex for heterotrophs than for algae. Generally, the presence of grazers tended to increase the biomass of bacteria, ciliates, and meiofauna. Thus, macrograzers were not generalist consumers of the entire community, but mainly reduced algae. Furthermore, the results suggested strong indirect effects of grazing, presumably through changes in nutrient supply and algal size structure. Nutrient enrichment had weak and inconsistent effects on bacterial, ciliate, and meiofaunal biomass. There was thus no complete propagation of bottom-up effects through the community, and strong internal feedback mechanisms within the periphyton mediated the effects of macroconsumers and nutrient enrichment.

Effect of grazing and water column nutrient supply on biomass and nutrient content of sediment microalgae

Field experiments were conducted to investigate the effects of grazing and nutrient supply on sediment microflora in a freshwater habitat (Lake Erken, Sweden) and at the brackish Baltic Sea coast (Vaddo, Sweden). The two sites were of similar productivity, but had contrasting herbivore composition. In a full-factorial experiment design, closed cages excluded macrozoobenthos (>1 mm) from sediment patches, whereas open cages allowed grazer access. The cage design applied here proved to successfully prevent in- and epifauna to access the sediment in closed cages. In half of the treatments, nutrients were added to the water-column by a slow-release fertilizer. The experiments were seasonally replicated four times at Vaddo and two times in Lake Erken. After 4–5 weeks, sediment cores were sampled and analyzed for chlorophyll, carbon, nitrogen and phosphorus. The benthic microalgae showed strong seasonal variation in biomass and internal nutrient content. At Vaddo, neither grazing nor nutrients affected the algal biomass significantly, but significant grazer effects were detected on C:N:P ratios. In Lake Erken, grazer presence reduced algal biomass by ca. 50%, whereas nutrients were without effect on biomass or on nutrient content. Compared to results from hard substrata at the same sites, sediment microflora was less affected by nutrients and grazing. This may be due to the harsh physico-chemical environment on sediments, to low grazer density at the coastal site and to low availability of water column nutrients to sediment microalgae. In our experiments, sand-dwelling microphytobenthic communities represented a highly dynamic assemblage, which, however, is less structured by biotic interactions than epilithic periphyton

Benthic diatom communities along pH and TP gradients in Hungarian and Swedish streams

Benthic diatoms are widely used in Europe to assess ecological status of running waters. The aim of this study was to develop models to predict average pH and total phosphorus (TP) for European stream monitoring using combined diatom data-sets from Hungary and Sweden. As first step, the relationship between the measured environmental variables and the distribution of the taxa was checked by classification by using cluster-analysis and CCA. Diatom data separated clearly along the main hydrogeochemical parameters (e.g. alkalinity, pH). Concerning nutrients, TP was the most determinant factor. Predictive value of the TP model (r = 0.96) is high only for the Hungarian data and the pH model (r = 0.97) can be used in both phycogeographical regions.

The impact of substrate and lake trophy on the biomass and nutrient status of benthic algae

Algal biomass, C:N:P (carbon:nitrogen:phosphorus) ratios and APA (biomass specific alkaline phosphatase activity) were measured in benthic algal communities on living substrates (mussels and macrophytes) and on rocks and stones (epilithon) in three lakes of different trophy. Benthic algal communities on living substrates had lower C:N:P ratios than epilithon, whereas algal biomass was highest on rocks and stones. Benthic algal biomass increased with the trophic level of a lake despite an increase of C:N:P ratios in the benthic community. The differences in C:N:P ratios and algal biomass between lakes of different trophy were higher on inert substrates than on macrophytes and mussels, probably because algae on living substrates could compensate a poor nutrient supply from lake water with substrate nutrients. However, the substrate was not, as expected, the most important nutrient supply in the oligotrophic lake, but in the eutrophic lake. Therefore, differences between inert and living substrates in a single lake were highest in the eutrophic lake. APA values of the oligotrophic lake were very high especially for benthic algae on stones, indicating an ability of the community to take up nutrients from organic sources. In conclusion, living substrates were an important nutrient source for benthic algae and the importance of this nutrient supply did not decrease with increasing lake trophy.

Gastropod grazers affect periphyton nutrient stoichiometry by changing benthic algal taxonomy and through differential nutrient uptake

Abstract Benthic grazers can affect the nutrient stoichiometry of periphyton through different pathways: 1) grazers can reduce periphyton biomass, thereby increasing the relative amount of nutrients for the remaining periphyton; 2) grazers can change benthic algal community composition toward grazer-resistant taxa that might differ in their C:N:P stoichiometry from less resistant benthic algae; 3) grazers can differentially recycle or incorporate nutrients, thereby changing periphyton nutrient content; and 4) mechanical removal of high-C detritus by grazers can decrease C:nutrient ratios. Nutrient-mediated grazer effects on periphyton nutrient stoichiometry depend on the relative availability of nutrients in the system in relation to the nutrients made available through grazing. We tested the relative importance of the different pathways under differing nutrient conditions in a 3-wk laboratory experiment in which grazer presence and dissolved N and P concentrations were manipulated. The effects of the gastropod grazer Viviparus viviparus (L.) on periphyton nutrient stoichiometry, algal taxonomy and biomass, and dissolved nutrients were determined with a 2 × 4 factorial design. Factors were grazers (+/–) and nutrient addition (ambient, +N, +P, and +NP). Grazers decreased periphyton C:P and N:P in the +N and +P treatments. Grazers mainly affected periphyton nutrient stoichiometry by changing benthic algal taxonomic composition because the percentage of mucilage-producing algae Chaetophora spp. was much higher in grazed than in ungrazed treatments. Mucilage has high C and N content, but low P content. Grazer stoichiometry indicated that grazers might have retained more P in the +N and +P treatment than in the other nutrient treatments. We conclude that nutrient addition interacted with grazer effects on periphyton nutrient stoichiometry and that grazer effects on periphyton nutrient stoichiometry can be explained only by considering the combined effects of nutrient availability, differential nutrient recycling by grazers, and grazer-induced changes in algal taxonomy.

Nutrient recycling-a strategy of a grazer community to overcome nutrient limitation

We examined in situ the fate of N in the epilithon and a grazer community in the N-limited rocky littoral of Lake Erken, Sweden. The grazer community consisted of the sessile caddisfly larva Tinodes waeneri (L.) plus its gallery. The gallery consisted of a web that is spun by the larva plus the microorganisms (mostly algae) closely associated with the web. We applied 15N in the field and recorded 15N uptake and retention time. The galleries took up 15N faster from the overlying water and retained 15N 1 wk longer than the surrounding epilithon. These results supported our hypothesis that effective N use through the Tinodes community by recycling may alleviate N limitation and enhance biomass production of the gallery algae.

Horizontal variation of biomass and C:N:P ratios of benthic algae in lakes

The horizontal variation of chlorophyll a (chl a) and C:N:P (carbon:nitrogen:phosphorus) ratio was estimated for benthic algal communities attached to living substrates (mussels and macrophytes) and to rocks and stones in three lakes of different trophy. Samples were taken in a nested hierarchical design with replicates separated by several cm, dm, 10 m, and km. The observed horizontal variation of chl a, C:N, and C:P ratios did not differ for horizontal scales, substrates, or lakes. Although the investigated lakes were quite unlike regarding nutrient status, light regime, or morphology, the patchiness was similar in all lakes. Moreover, patchiness was also similar on stones, macrophytes and mussels, although those substrates differed in longevity and surface structure. Similar patchiness regardless of scale, substrate, or sampled lake, implies the possibility of using an optimal sampling design calculated for one lake and substrate also in other lakes and on other substrates.

Alkaline phosphatase activity in periphyton communities

Ectoenzymes are actively secreted outside the cytoplasmatic membrane and remain bound to a cell surface. They are believed to be synrhesised constitutively at a low level and to be under a catabolic repression/derepression conrrol mechanism from the end product. Furthermore, synrhesis can be activated by inductors, which indicate substrate availabiliry. Alkaline phosphatases (phosphomonoesterases) are among the most widely distributed ectoenzymes in aquatic environments (CHROST 1991). During recent decades, the development of fluorometric techniques to measure enzymatic activiry, as well as a growing inrerest in phosphorus turnover, has promoted studies by ecologists. Alkaline phosphatases have been found to be associated with algal, zooplankton and bacterial cells. Potenrial (measured with a commercial subsrrate) alkaline phosphatase activity has been used as an indicator of phosphorus deficiency in algal cultures and natural freshwater phytoplankton communities, since suggested by FITZGERALD & NELSON (1966). They reported up to 25 times higher activiry in Plimited algae. The application of potential alkaline phosphatase activity as a P-deficiency indicator to freshwater benthic algae has been much more restricted than its use in the pelagic system. FREEMAN ( 1986) suggested seasonal P limitation of periphyton based on high alkaline phosphatase activiry. RosEMOND (1993) found decreased alkaline phosphatase activiry with nutrient enrichment. BuRKHOLDER & WETZEL (1990) used alkaline phosphatase activiry to demonstrate the role of rwo macrophyte species as a source of phosphorus for epiphytes, by comparing activities berween the natural substrata and artificial plants. CATTANEO & KALFF (1979) consistently measured significanrly higher alkaline phosphatase activiry per unit of algal chlorophyll a for epiphytes from artificial planrs. The aim of this work was to demonstrate the use of potenrial alkaline phosphatase activiry as a P-deficiency indicator in freshwater benthic algae by measuring periphyton from three lakes and one wedand of very different trophic levels and with widespread phosphorus availabiliry. Additionally, seasonal variations were followed.