Direct and indirect effects of zooplankton on algal composition in in situ grazing experiments

To examine both direct and indirect effects of macrozooplankton on phytoplankton species in Lake Biwa, we conducted in situ grazer-gradient experiments under different nutrient levels in summer, when Daphnia galeata dominated, and in autumn, when Eodiaptomus japonicus dominated. The experiments revealed that grazing pressure on phytoplankton was highly dependent on zooplankton species composition. Smaller phytoplankton species such as Stephanodiscus carconensis were more grazed when D. galeata was abundant, whereas large colonial diatom species such as Aulacoseira granulata were preferentially grazed when E. japonicus dominated. In addition, indirect effect of macrozooplankton through nutrient regeneration was suggested, although the magnitude of nutrient regeneration effects seemed to differ between D. galeata and E. japonicus. Specifically, growth rates of Sphaerocystis schroeteri were stimulated more by E. japonicus than by D. galeata. Macrozooplankton also enhanced the growth rates of colonial cyanobacteria such as Microcystis incerta, probably through decreasing the density of microzooplankton grazers (ciliates and rotifers). The results suggest that the effects of large zooplankton on phytoplankton populations are species-specific and cannot be understood without consideration of changes in abundance of other components of plankton communities.     

Qualitative importance of the microbial loop and plankton community structure in a eutrophic lake during a bloom of cyanobacteria

Plankton community structure and major pools and fluxes of carbon were observed before and after culmination of a bloom of cyanobacteria in eutrophic Frederiksborg Slotssø, Denmark. Biomass changes of heterotrophic nanoflagellates, ciliates, microzooplankton (50 to 140 μm), and macrozooplankton (larger than 140 μm) were compared to phytoplankton and bacterial production as well as micro- and macrozooplankton ingestion rates of phytoplankton and bacteria. The carbon budget was used as a means to examine causal relationships in the plankton community. Phytoplankton biomass decreased and algae smaller than 20 μm replacedAphanizomenon after the culmination of cyanobacteria. Bacterial net production peaked shortly after the culmination of the bloom (510 μg C liter^?1 d^?1 and decreased thereafter to a level of approximately 124 μg C liter^?1 d^?1. Phytoplankton extracellular release of organic carbon accounted for only 4–9% of bacterial carbon demand. Cyclopoid copepods and small-sized cladocerans started to grow after the culmination, but food limitation probably controlled the biomass after the collapse of the bloom. Grazing of micro- and macrozooplankton were estimated from in situ experiments using labeled bacteria and algae. Macrozooplankton grazed 22% of bacterial net production during the bloom and 86% after the bloom, while microzooplankton (nauplii, rotifers and ciliates larger than 50 μm) ingested low amounts of bacteria and removed 10–16% of bacterial carbon. Both macro-and microzooplankton grazed algae smaller than 20 μm, although they did not control algal biomass. From calculated clearance rates it was found that heterotrophic nanoflagellates (40–440 ml^?1) grazed 3–4% of the bacterial production, while ciliates smaller than 50 μm removed 19–39% of bacterial production, supporting the idea that ciliates are an important link between bacteria and higher trophic levels. During and after the bloom ofAphanizomenon, major fluxes of carbon between bacteria, ciliates and crustaceans were observed, and heterotrophic nanoflagellates played a minor role in the pelagic food web.     

Top-down control of natural phyto- and bacterioplankton prey communities by Daphnia magna and by the natural zooplankton community of the hypertrophic Lake Blankaart

Biomanipulation, through the reduction of fish abundance resulting in an increase of large filter feeders and a stronger top-down control on algae, is commonly used as a lake restoration tool in eutrophic lakes. However, cyanobacteria, often found in eutrophic ponds, can influence the grazing capacity of filter feeding zooplankton. We performed grazing experiments in hypertrophic Lake Blankaart during two consecutive summers (1998, with and 1999, without cyanobacteria) to elucidate the influence of cyanobacteria on the grazing pressure of zooplankton communities. We compared the grazing pressure of the natural macrozooplankton community (mainly small to medium-sized cladocerans and copepods) with that of large Daphnia magna on the natural bacterioplankton and phytoplankton prey communities. Our results showed that in the absence of cyanobacteria, Daphnia magna grazing pressure on bacteria was higher compared to the grazing pressure of the natural zooplankton community. However, Daphnia grazing rates on phytoplankton were not significantly different compared to the grazing rates of the natural zooplankton community. When cyanobacteria were abundant, grazing pressure of Daphnia magnaseemed to be inhibited, and the grazing pressure on bacteria and phytoplankton was similar to that of the natural macrozooplankton community. Our results suggest that biomanipulation may not always result in a more effective top-down control of the algal biomass.     

Zooplankton community size structure and taxonomic composition affects size-selective grazing in natural communities

The body size of an individual zooplankton is well related to its grazing rate and to the range of particle sizes it can ingest, and since cladocerans and copepods feed differently, they follow different relationships. Based on these general patterns in individual organisms, we tested whether the size structure and taxonomic composition of more complex natural zooplankton communities are related to their in situ grazing rate and to the range of algal sizes they graze. We compared community grazing rates on individual algal taxa in two communities dominated by small cladocerans, three communities dominated by large cladocerans and three copepod-dominated communities. Small algae were usually grazed most intensively, but grazing rates were poorly related to algal size alone. The range in size of grazed algae increased with increasing mean zooplankton body size, but differed systematically with their taxonomic composition. Communities dominated by Ceriodaphnia or Holopedium grazed a narrower size range of algae [maximum greatest axial length dimension (GALD)=16–36 μm)] than communities with large biomasses of Bosmina or Daphnia (maximum GALD=28–78 μm). Copepod-dominated communities followed the same general relationship as cladocerans. Daphnia-dominated communities grazed the broadest range of algal sizes, and their total grazing rates were up to 2.4 times their grazing rates on small (<35 μm) “highly edible” algae, a difference of similar magnitude to those found in successful trophic cascade biomanipulations. Received: 31 March 1998 / Accepted: 19 October 1998     

Trophic interactions within the microbial food web in a tropical floodplain lake (Laguna Bufeos, Bolivia)

Whether the primary role of bacterioplankton is to act as "remineralizers" of nutrients or as direct nutritional source for higher trophic levels will depend on factors controlling their production and abundance. In tropical lakes, low nutrient concentration is probably the main factor limiting bacterial growth, while grazing by microzooplankton is generally assumed to be the main loss factor for bacteria. Bottom-up and top-down regulation of microbial abundance was studied in six nutrient limitation and dilution gradient-size fractionation in situ experiments. Bacteria, heterotrophic nanoflagellates (HNF), ciliates and rotifers showed relatively low densities. Predation losses of HNF and ciliates accounted for a major part of their daily production, suggesting a top-down regulation of protistan populations by rotifers. Phosphorus was found to be strongly limiting for bacterial growth, whereas no response to enrichment with Nitrogen or DOC was detected. HNF were the major grazers on bacteria (g-0.43 d(-1)), the grazing coefficient increased when ciliates were added (g- 0.80 d(-1)) but decreased when rotifers were added (g- 0.23 d(-1)) probably due to nutrient recycling or top-down control of HNF and ciliates by rotifers.     

Microbial food web in a large shallow lake (Lake Balaton, Hungary)

Seasonal variations of phyto-, bacterio- and colourless flagellate plankton were followed across a year in the large shallow Lake Balaton (Hungary). Yearly average chlorophyll-a concentration was 11 µg 1^–1, while the corresponding values of bacterioplankton and heterotrophic nanoflagellate (HNF) plankton biomass (fresh weight) were 0.24 mg 1^–1 and 0.35 mg 1^–1, respectively. About half of planktonic primary production was channelled through bacterioplankton on the yearly basis. However, there was no significant correlation between phytoplankton biomass and bacterial abundance. Bacterial specific growth rates were in the range of 0.009 and 0.09 h^–1, and ended to follow the seasonal changes in water temperature. In some periods of the year, predator-prey relationships between the HNF and bacterial abundance were obvious. The estimated HNF grazing on bacteria varied between 3% and 227% of the daily bacterial production. On an annual basis, 87% of bacterial cell production was grazed by HNF plankton.     

Experimental demonstration of the roles of bacteria and bacterivorous protozoa in plankton nutrient cycles

We have used a model food chain composed of a natural bacterial assemblage, a pennate diatom and a bacterivorous microflagellate to investigate the factors controlling the relative importance of bacteria and protozoa as sources for regenerated nitrogen in plankton communities. In bacterized diatom cultures in which diatom growth was nitrogen-limited, the carbon:nitrogen (C:N) ratio of the bacterial substrate greatly affected which population was responsible for the uptake of nitrogen. When nitrogen was added as NH ₄ ⁺ and the cultures were supplemented with glucose, the bacteria competed successfully with the algae for NH ₄ ⁺ and prevented the growth of algae by rapidly assimilating all NH ₄ ⁺ in the cultures. Bacterivorous protozoa inoculated into these cultures grazed the bacterial population and remineralized NH ₄ ⁺ , thus relieving the nitrogen limitation of algal growth and allowing an increase in algal biomass. In contrast, bacteria in cultures supplemented with the amino acid glycine (C:N = 2) were major remineralizers of nitrogen, and the influence of protozoan grazing was minimal. We conclude that the relative importance of bacteria and protozoa as nutrient regenerators in the detrital food loop is dependent largely on the overall carbon:nutrient ratio of the bacterial substrate. The role of bacterivorous protozoa as remineralizers of a growth-limiting nutrient is maximal in situations where the carbon:nutrient ratio of the bacterial substrate is high.     

The role of light availability and herbivory on algal responses to nutrient enrichment in a riparian wetland,Alaska

We investigated how the relative availability of solar radiation in the presence or absence of grazing alters the ability of benthic algae to respond to nutrient enrichment in an Alaskan marsh. We used a factorial mesocosm experiment that included nutrient enrichment (enriched or control), grazing (grazed or ungrazed), and light (unshaded or shaded) to simulate shading by macrophytes early and late in the growing season, respectively. We found stronger effects of grazers and nutrients compared to light on benthic algal biomass and taxonomic composition. Algal biomass increased in nutrient‐enriched treatments and was reduced by grazing. Shading did not have an effect on algal biomass or taxonomic composition, but the concentration of chl a per algal biovolume increased with shading, demonstrating the ability of algae to compensate for changes in light availability. Algal taxonomic composition was more affected by grazer presence than nutrients or light. Grazer‐resistant taxa (basal filaments of Stigeoclonium) were replaced by diatoms (Nitzschia) and filamentous green algae (Ulothrix) when herbivores were removed. The interacting and opposing influences of nutrients and grazing indicate that the algal community is under dual control from the bottom‐up (nutrient limitation) and from the top‐down (consumption by herbivores), although grazers had a stronger influence on algal biomass and taxonomic composition than nutrient enrichment. Our results suggest that low light availability will not inhibit the algal response to elevated nutrient concentrations expected with ongoing climate change, but grazers rapidly consume algae following enrichment, masking the effects of elevated nutrients on algal production.     

Zooplankton biomass distribution patterns along the western Antarctic Peninsula (December 2002)

The phytoplankton [chlorophyll a (Chl a)], microzooplankton,mesozooplankton and macrozooplankton biomass and distributionwere studied as part of a multidisciplinary project (Tempano)along the Antarctic Peninsula during December 2002. Even thoughthe summer phytoplankton bloom was not yet developed in thearea, autotrophs dominated the plankton biomass. Phytoplanktonvertical distribution was, in general, homogeneous in the upper40–50 m of the water column, further decreasing with depth.Protozoans showed low biomass; their contribution to the totalplankton being one order of magnitude lower than that of autotrophs.The vertical distribution of protozoans was variable among stationswith marked peaks at depths ranging from 30 to 80 m. Mesozooplankton-integratedbiomass was generally low, although there was a notable increasesouthward near the ice marginal zone. Macrozooplankton distributionwas more variable without any clear zonal distribution pattern.The vertical distribution of meso- and macrozooplankton (>4mm) biomass showed clear peaks of abundance comprising differentspecies depending on the geographical area. Our biomass distributiondata suggest a food-web scenario in which macrozooplankton arepreying on mesozooplankton populations only in the northernerstations, and mesozooplankton are, in their turn, shaping theabundance of the emerging populations of microzooplankton. Phytoplankton,on the other hand, seem to be hardly controlled by grazing activity.     

Planktonic food chains of a highly humic lake

The development and metabolism of epilimnetic plankton from a highly humic lake was followed in late summer, when the predominant zooplankton species, Daphnia longispina, was very abundant (ca. 200 ind. l^?1). The experiment was made in two tanks: one with an unaltered plankton assemblage and one with larger zooplankton removed. The scarce phytoplankton community was also simple, consisting mainly of one Cryptomonas and two Mallomonas species. The abundance and species composition of smaller plankton was heavily influenced by grazing of Daphnia. In particular, the biomass, of heterotrophic flagellates increased after the removal of Daphnia. The biomass and production of bacterioplankton were not affected, and remained several times higher than that of phytoplankton. Bacterial production and grazing on bacteria were balanced, and when Daphnia was removed its grazing activity was compensated by flagellates. The removal of Daphnia did not affect the respiration or community net production of plankton. Among organisms smaller than zooplankton, bacteria seemed to be responsible for most of the respiration. The community net production was consistently negative even at the water surface, indicating an allochthonous carbon source. The results suggest that phytoplankton primary production was insufficient for the secondary production in the epilimnetic water of the study lake. The food requirements of bacteria and zooplankton, as well as of flagellates, each exceeded that supplied by phytoplankton primary production. The simple food chains in this experiment made it possible to reveal the functioning of the community so completely that dissolved organic matter is certainly comparable to or exceeds the importance of phytoplankton primary production as an energy and carbon source for food webs in this humic lake.     

Elemental composition (C:N:P) and growth rates of bacteria and Rhodomonas grazed by Daphnia

The elemental composition and growth rate of Rhodomonas andheterotrophic bacteria were studied in batch cultures in thepresence and absence of Daphnia and at two different levelsof phosphorus limitation. The elemental content of single cellswas measured with X-ray microanalysis. Simultaneously, dilutionexperiments were performed in order to estimate grazing losses,growth rates and dominant nutrient sources for bacteria andRhodomonas. The phosphorus:carbon (P:C) ratios of the bacteriawere generally higher in the experiment with the stronger Plimitation of the system. High P:C ratios were taken as an indicationthat bacteria were carbon limited. The presence of Daphnia resultedin a further increase in bacterial P:C ratios and increasedspecific growth rates. Thus, grazing increased the availabilityboth of inorganic nutrients and organic substrates, stimulatingthe growth of the bacteria. P:C ratios of Rhodomonas decreasedwith increasing P limitation of the system. Only at strong Plimitation did the presence of Daphnia result in increased P:Cratios of Rhodomonas compared with the control without daphnids.This study shows that the elemental content and growth rateof heterotrophic bacteria and Rhodomonas are influenced by grazingand nutrient regeneration by daphnids. The response is dynamicand depends on the level of nutrient limitation of the system. Present address: Department of Microbiology, University of BergenJahnebakken 5, NO-5020 Bergen, Norway     

How important is bacterial carbon to planktonic grazers in a turbid, subtropical lake?

This study examined the relative contributions of bacterialand phytoplankton production to the pelagic carbon flow of LakeOkeechobee, a large and shallow subtropical lake. Due to thepredominance of cyanobacteria in this lake, we hypothesizedthat bacterial carbon flow would be larger than phytoplanktoncarbon flow to grazers. Using epifluorescent and light microscopyand radiotracer techniques, we measured the carbon biomass ofplanktonic functional groups and carbon flow between these groups.The functional groups that we used in this study included: picophytoplankton,autotrophic nanoflagellates (ANAN), microphytoplankton, bacteria,heterotrophic nanoflagellates (HNAN), ciliates, microzooplankton(rotifers and copepod nauplii) and macrozooplankton (cladocerans,copepodites and adult copepods). Microphytoplankton dominatedthe carbon biomass of all plankton, whereas the calanoid copepod,Diaptomus, dominated the carbon biomass of the grazers. Phytoplanktoncarbon flow often was higher than bacterial carbon flow to grazers;however, bacterial carbon constituted a large percentage ofthe total carbon flow to grazers (33.7 ± 22.4%). Bacterialcarbon provided roughly one quarter of the carbon flow to macrozooplankton(27.1 ± 25.4%), whereas it provided half of the carbonflow to microzooplankton (57.4 ± 20.3%) and to protozoans(47.2 ± 25.8%). These results suggest that microbialpathways play an important role in the energetics of subtropicallake plankton communities. Although microbial loop pathwaysare important in many systems, direct bacterial carbon flowto macrozooplankton also may be important in copepod- and cyanobacteria-dominatedlakes.     

The importance of crustacean zooplankton in structuring rotifer and phytoplankton communities; an enclosure study

During the spring clear-water phase of 1993, an enclosure experimentwas performed in the mesotrophic Schöhsee (Plön, FRG)in order to assess the impact of crustacean zooplankton on therotifer and phytoplankton community. Among the crustacean plankton,calanoid and cyclopoid cope-pods were abundant, but Daphnia‘longispina’ reached the highest densities in thisexperiment. The colonial rotifer Conochilus unicomis was notaffected by crustacean plankton. The two most abundant species,Synchaeta peclinata and Keratella cochlearis, increased exponentiallywhen macrozooplankton had been excluded from the enclosures,but did not increase when crustaceans were present. Birth anddeath rates of K.cochlearis could be reliably determined inthis field experiment, suggesting that this rotifer specieswas mainly controlled by exploitative competition rather thanby mechanical interference or predation. Daphnia ‘longispina’generally grazed selectively on the smaller ciliates and algae,thus depriving the rotifers of their phytoplankton resources.The dominant alga, the chrysophycean Dinobryon, increased, whethercrustaceans were present or not, but appeared to be grazed uponto a certain extent despite its considerable cell size and colonialorganization.     

Contrasting ‘Top-Down’ Effects of Crustacean Zooplankton Grazing on Bacteria and Phytoflagellates

The combined effects of grazing and nutrient regeneration by Daphnia and Eudiaptomus on the growth of Rhodomonas and heterotrophic bacteria was assessed experimentally. The responses of Rhodomonas and bacteria to the grazers were measured as net specific growth rate over the entire experimental periods, as well as production and specific production at the end of the experiments. Both zooplankton species had a negative effect on Rhodomonas net specific growth rate due to grazing and a positive effect on specific primary production due to nutrient regeneration. Daphnia had no effect on bacterial net specific growth rate, bacterial production or specific bacterial production in one of two experiments. In the other experiment, however, both bacterial growth rate and production decreased as a result of grazing. Furthermore, Daphnia had a negative effect on specific bacterial production, but Eudiaptomus had a positive effect on all bacterial parameters due to nutrient regeneration, probably of phosphorus. Positive effects of copepods on bacterial growth has previously been attributed to trophic cascades via protozoa. However, the present experiments show that regeneration of nutrients, especially phosphorus, may account for a large part of the stimulation of bacterial growth.     

The effect of different zooplankton grazing patterns resulting from diel vertical migration on phytoplankton growth and composition: a laboratory experiment

Diel vertical migration (DVM) of herbivorous zooplankton is a widespread behavioural phenomenon in freshwater ecosystems. So far only little attention has been paid to the impact of DVM on the phytoplankton community in the epilimnion. Some theoretical models predict that algal population growth in the epilimnion should depend on the herbivores migration and grazing patterns: even if migrating zooplankton consume the same total amount of algae per day in the epilimnion as non-migrating zooplankton, nocturnal grazing should result in enhanced algal growth and favour algal species with high intrinsic growth rates over species with lower intrinsic growth rates. To test these hypotheses we performed experiments in which several algal species were confronted with different feeding regimes of Daphnia. In the experiments algal growth did not only depend on the absolute time of grazing but was comparatively higher when grazing took place only during the night, even when the grazing pressure was the same. Furthermore, algal species with higher intrinsic growth rates had higher advantages when being grazed upon only discontinuously during the night than algal species with a smaller intrinsic growth rate. The grazing pattern itself was an important factor for relative algal performance.     

Influence of two Daphnia species on summer phytoplankton assemblages from eutrophic lakes

We conducted grazing experiments to test whether larger-bodiedDaphnia pulicaria have a different effect from smaller-bodiedDaphnia galeata mendotae on the composition of summer algalassemblages in eutrophic lakes. Three separate cubitainer experimentswere run for 5 days in a replicated factorial design utilizingtwo algal community types and the two Daphnia species. Inorganicphosphorus and nitrogen were added to prevent nutrient limitationof the algae. Both edible and inedible size fractions of chlorophylla increased in cubitainers without Daphnia spp. Grazer additionusually resulted in a reduction in edible chlorophyll; reductionswere greater in D.pulicaria cubitainers. Grazing by Daphniaspp. on presumed inedible chlorophyll was variable. Algal sizewas not always a good predictor of grazeability. The resultsof this study indicate that D.pulicaria, because of its greaterfiltration potential and ability to ingest larger particles,provides a stronger control on inedible-sized algae when comparedto equal numerical densities of D.g.mendotae. However, Aphanizomenonincreased as a response to heavy grazing pressure by D.pulicariaon other algal species. This suggests that biomanipulation effortsthat promote large-bodied Daphnia may not produce desirableresults if nutrient inputs remain high.     

Predaceous insects may limit algal grazers in high-nutrient environments: implications for wastewater remediation in open bioreactors

Algal biofuel has potential as a source of renewable fuel and a tool for wastewater remediation. Open algal bioreactors fertilized with wastewater can have net energy gain but are vulnerable to colonization by algal grazers. However, colonizing predaceous insects may limit grazer impacts on algae. Here, we investigate the effects of grazers, predators, and invading algae species on algal production and community structure in high-nutrient environments. First, we grew diverse algal assemblages in treated municipal wastewater in a greenhouse with Daphnia grazers and different insect predators that were added experimentally. When Daphnia were present without predators, they eliminated suspended algae. But, dragonfly larvae [Odonata: Libellulidae] and backswimmers [Hemiptera: Notonectidae], but not larval diving beetles [Coloeoptera: Dytiscidae], suppressed Daphnia allowing suspended algae to persist. Second, we grew Chlorella algae in field tanks that were open or protected from natural invertebrate colonization and half the tanks received wild-collected plankton in a factorial design. Mosquito larvae [Culex sp.] readily colonized open tanks and reduced algal mass and dissolved phosphorus concentrations. Colonist addition to open tanks shifted algal functional and taxonomic composition but did not impact suspended algal production. Our study indicates that large numbers of grazer individuals can rapidly colonize open bioreactors. Experimentally added and naturally colonizing grazers altered algal community structure and reduced algal standing crops but may also aid in nutrient removal from wastewater-fed bioreactors. Effective operation of open algal bioreactors must consider cultivated algae species’ vulnerability to competition and local grazers as well as the ability of potential predators to both naturally disperse into bioreactors and to control grazers.     

Plankton community of a polyhumic lake with and without Daphnia longispina (Cladocera)

The impact of Daphnia longispina (Cladocera) on the plankton food web was studied in a polyhumic lake where this species comprised almost all zooplankton biomass. Plastic enclosures (volume 7 m³) were inserted into the lake retaining the initial water stratification except that in one enclosure zooplankton was removed. After the removal of Daphniaa rotifer, Keratella cochlearis, ciliates and heterotrophic nanoflagellates increased markedly and the density and biomass of bacteria decreased. Edible algal species, Cryptomonas rostratiformisand three small chrysophytes,Ochromonas, Pedinella and Spinifermonas, took advantage of the removal of Daphnia, while more grazing-resistant species declined. In spite of the changes in the species composition of phytoplankton, the removal of Daphnia did not affect the biomass, primary production or respiration of plankton. The results implied that the density of heterotrophic flagellates and ciliates was controlled by Daphnia, but in its absence the former took its role as the bacterial grazers.     

Ciliates are the Dominant Grazers on Pico- and Nanoplankton in a Shallow,Naturally Highly Eutrophic Lake

Abundance and biomass of the microbial loop members [bacteria, heterotrophic nanoflagellates (HNF), and ciliates] were seasonally measured in the naturally eutrophic and shallow (2.8 mean depth) Lake Võrtsjärv, which has a large open surface area (average 270 km²) and highly turbid water (Secchi depth <1 m). Grazing rates (filter feeding rates) on 0.5-, 3-, and 6-μm-diameter particles were measured to estimate pico- and nanoplankton grazing (filter feeding) by micro- and metazooplankton. Among grazers, HNF had a low abundance (<50 cells mL^?1) and, due to their low specific filtering rates, they only grazed a minor fraction of the bacterioplankton (≤4.2% of total grazing). Ciliates were relatively abundant (≤158 cells mL^?1) and, considering their high specific feeding rates, were able to graze more than 100% of the bacterial biomass production in the open part of the lake, whereas the average daily grazing accounted for 9.3% of the bacterial standing stock. Ciliates were potentially important grazers of nanoplanktonic organisms (on average, approximately 20% of the standing stock of 3-μm-size particles was grazed daily). Metazooplankton grazed a minor part of the bacterioplankton, accounting for only 0.1% of standing stock of bacteria. Grazing on nanoplankton (3–6 μm) by metazooplankton was higher (0.4% of standing stock). The hypothesis is proposed that ciliates dominate due to a lack of top–down regulation by predators, and HNF have a low abundance due to strong grazing pressure by ciliates.     

Algal propagule banks modify competition, consumer and resource control on Baltic rocky shores

We hypothesized that supply from macroalgal propagule banks may influence the relative abundance of annual and perennial algae and that this may alter the effects of grazers and nutrients on species composition. In a factorial field experiment in the Baltic Sea littoral system we tested the effects of manipulating propagule banks, the abundance of crustacean and gastropod grazers, and nutrient supply on recruitment and growth of macroalgae over a year. Moreover, we determined seasonal patterns of macroalgal propagule dispersal at the experimental site and quantified algal abundance and recruitment at 25 locations throughout the Baltic Sea. Experimental manipulations had minor effects on adults of the dominating perennial alga, Fucus vesiculosus. Instead, we found that species composition was determined by processes operating at early life stages. Propagule supply from a propagule bank strongly favored the fast-growing annual alga Enteromorpha spp. which then blocked settlement and recruitment of Fucus. Grazers reduced the abundance of annual algae and indirectly favored Fucus recruitment. There was an apparent trade-off between gains from the propagule bank and losses to herbivory in five of seven colonizing species. Nutrient enrichment overrode grazer control of annual algae and accelerated the decline of Fucus only when annual algae had already achieved high densities through the propagule bank. Corroborating the experimental findings, field surveys across the Baltic showed that Fucus recruit densities can be predicted from the cover of annual algae during the period of Fucus reproduction and settlement. Recruitment inhibition by annual algae, which is driven by the abundance of annuals in the propagule bank, increasing nutrient levels, and declining consumer control, is suggested as a mechanistic explanation of the current decline of perennial algae in the Baltic Sea.