Oligotrophic Communities Research Articles

Mycorrhizal Fungi Reclamation Promotes Stoichiometric Homeostasis of Re-Vegetation Types and Affects Soil Bacterial Function in Mining Subsidence of Northern Loess Plateau

Re-vegetation types and mycorrhizal fungi reclamation play a vital role in the improvement of soil quality in the mining subsidence of the northern Loess Plateau. However, the effects of re-vegetation types and mycorrhizal fungi reclamation on plant stoichiometric homeostasis, soil bacterial communities and functional characteristics are still not understood well but are vital for mining green construction. Based on the fact that mycorrhizal fungi reclamation has been implemented for more than 10 years (inoculation with arbuscular mycorrhizal fungi (AMF) and control), we examined five re-vegetation types with different C:N:P stoichiometry in the roots, leaves and calculated homeostasis. Meanwhile, second-generation sequencing technology was used to measure soil bacterial communities and functional characteristics to further reveal the relationships between soil factors and bacteria that drive plant stoichiometry and homeostasis in the biological reclamation area of coal mining subsidence. Our results indicated that plant N:P ratio in the leaves of all re-vegetation types was less than 14, with the highest ratio observed in A. fruticosa (nitrogen-fixing plants), showing that re-vegetation growth was limited by the availability of nitrogen. Only leaves in AMF-inoculated plants were categorized as ‘homeostatic’, while inoculation with AMF in both leaves and roots could alleviate nitrogen restriction and improve ecological stoichiometric homeostasis. The dominant phylum was Proteobacteria, followed by Actinobacteria, Acidobacteria, accounting for 69.92%–73.22% of all bacterial species and 82% with Chloroflexi. Soil copiotrophic community (Proteobacteria) in the AMF inoculation area was higher than those in the control area under all re-vegetation types, while the oligotrophic community (Acidobacteria) was lower than the control. Further analysis showed that soil TP, SOC, C:N and HD played vital roles in shifting the soil bacteria community. Soil stoichiometry and AMF affect microbial composition. These results indicated that the re-vegetation types and mycorrhizal fungi reclamation could shift bacterial homogeneity. Hence, our results expound that mycorrhizal fungi reclamation could optimize the ecological strategies of reclaimed vegetation, alleviate N-limitations in plants, improve endogenous stability and promote the ecological function of soil bacteria, which provided theoretical bases for further understanding and application of green restoration and sustainable development in the mining subsidence of the northern Loess Plateau.

Microbial communities associated with sinking particles across an environmental gradient from coastal upwelling to the oligotrophic ocean

Complex interactions between microbial communities in the epipelagic and mesopelagic ocean drive variability in sinking carbon flux as part of the biological carbon pump. To investigate the relative contribution of dominant prokaryotic, protistan, and phytoplankton taxa to sinking particles, we used 16S/18S rRNA gene sequencing of particles collected in sediment traps and water column communities. Comparisons were done across a coastal upwelling-to-oligotrophic environmental gradient in the southern California Current Ecosystem to assess effects of natural environmental variability on taxon-specific contributions to sinking particle export. Particle-associated microbial assemblages differ from ambient mixed-layer communities. Gammaproteobacteria, dinoflagellates, and rhizarians were dominant microbes associated with sinking particles at all sampling locations, with diatoms increasing significantly at the inshore mesotrophic site. Parasitic groups, syndiniales and apicomplexans, were also major particle-associated taxa. Relative sequence abundance on exported particles from the mesotrophic inshore site more closely resembled the water-column dominants (especially diatoms) found above, while oligotrophic communities exhibited greater dissimilarity between the microbial communities on sinking particles and in the mixed layer. Protists generally showed greater similarity between mixed-layer and sinking particle communities than prokaryotes. Synechococcus was over-represented on sinking particles (relative to eukaryotic phytoplankton and other cyanobacteria) only in oligotrophic areas. Diatoms were the only phytoplankton group consistently over-represented on sinking particles across the region. Our results highlight important variability in microbial contributions to export that seem to align with differences in dominant grazing pathways. At the upwelling influenced site where export was primarily due to the fecal pellets of large copepods, diatoms were dominant contributors to export and protistan communities on sinking particles resembled those in the water column above. When doliolids were abundant, dinoflagellates contributed substantially to export, while the dominant picoeukaryote (Ostreococcus) was only a minor contributor. In oligotrophic areas, where grazing pathways were dominated by protists and sinking particles were primarily amorphous aged aggregates, there was little similarity between mixed layer and sinking particle communities.

Effects of nutrient enrichments on oligotrophic phytoplankton communities: a mesocosm experiment near Hawai‘i, USA

A large-volume mesocosm-based nutrient perturbation experiment was conducted off the island of Hawai‘i, USA, to investigate the response of surface ocean phytoplankton communities to the addition of macronutrients, trace metals, and vitamins and to assess the feasibility of using mesocosms in the open ocean. Three free-drifting mesocosms (~60 m3) were deployed: one mesocosm served as a control (no nutrient amendments); a second (termed +P) was amended with nitrate (N), silicate (Si), phosphate (P), and a trace metal + vitamin mixture; and a third (termed -P) was amended with N, Si, and a trace metal + vitamin mixture but no P. These mesocosms were unreplicated due to logistical constraints and hence differences between treatments are qualitative. After 6 d, the largest response of the phytoplankton community was observed in the +P mesocosm, where chlorophyll a and 14C-based primary production were 2-3× greater than in the -P mesocosm and 4-6× greater than in the control. Comparison between mesocosm and ‘microcosm’ incubations (20 l) revealed differences in the magnitude and timing of production and marked differences in community structure with a reduced response of diatoms in microcosm treatments. Notably, we also observed pronounced declines in Prochlorococcus populations in all treatments, although these were greater in microcosms (up to 99%). Overall, this study confirmed the feasibility of deploying free-drifting mesocosms in the open ocean as a potentially powerful tool to investigate ecological impacts of nutrient perturbations and constitutes a valuable first step towards scaling plankton manipulation experiments.

Растительность класса Scheuchzerio–Caricetea fuscae Tx. 1937 в районе горного массива Янганапэ (восточный макросклон Полярного Урала)

Растительность класса Scheuchzerio–Caricetea fuscae Tx. 1937 в районе горного массива Янганапэ (восточный макросклон Полярного Урала)

Phytoplankton growth and stoichiometric responses to warming, nutrient addition and grazing depend on lake productivity and cell size.

Global change involves shifts in multiple environmental factors that act in concert to shape ecological systems in ways that depend on local biotic and abiotic conditions. Little is known about the effects of combined global change stressors on phytoplankton communities, and particularly how these are mediated by distinct community properties such as productivity, grazing pressure and size distribution. Here, we tested for the effects of warming and eutrophication on phytoplankton net growth rate and C:N:P stoichiometry in two phytoplankton cell size fractions (<30 µm and >30 µm) in the presence and absence of grazing in microcosm experiments. Because effects may also depend on lake productivity, we used phytoplankton communities from three Dutch lakes spanning a trophic gradient. We measured the response of each community to multifactorial combinations of temperature, nutrient, and grazing treatments and found that nutrients elevated net growth rates and reduced carbon:nutrient ratios of all three phytoplankton communities. Warming effects on growth and stoichiometry depended on nutrient supply and lake productivity, with enhanced growth in the most productive community dominated by cyanobacteria, and strongest stoichiometric responses in the most oligotrophic community at ambient nutrient levels. Grazing effects were also most evident in the most oligotrophic community, with reduced net growth rates and phytoplankton C:P stoichiometry that suggests consumer‐driven nutrient recycling. Our experiments indicate that stoichiometric responses to warming and interactions with nutrient addition and grazing are not universal but depend on lake productivity and cell size distribution.

Zooplankton communities and Bythotrephes longimanus in lakes of the montane region of the northern Alps

Lakes in the Alps represent a considerable fraction of nutrient-poor lakes in Central Europe, with unique biodiversity and ecosystem properties. Although some individual lakes are well studied, less knowledge is available on large-scale patterns essential to general understanding of their functioning. Here, we aimed to describe crustacean zooplankton communities (Cladocera, Copepoda) and identify their environmental drivers in the pelagic zone of 54 oligotrophic lakes in the montane region of the Alps (400–1200 m) in Austria, Germany, and Switzerland, covering a spatial scale of 650 km. Moreover, we aimed to provide data on the distribution and ecological requirements of the North American invader Bythotrephes longimanus in its Central European native range. Communities were mainly dominated by widespread species typical of lowland habitats, and only a few true specialists of oligotrophic alpine lakes were present. The most frequent taxa were the Daphnia longispina complex and Eudiaptomus gracilis, with 48 and 45 occurrences, respectively. Species richness decreased with altitude and increased with lake area. The main structuring factors of community composition were chlorophyll a concentration and depth, which drove an apparent separation of mesotrophic and oligotrophic communities. Bythotrephes had 13 occurrences, showing a preference for deep oligotrophic lakes. Its presence was not coupled with lower crustacean species richness, as was repeatedly observed in North America. Additionally, it frequently co-occurred with the other large predatory cladoceran, Leptodora kindtii. B. longimanus might be considered a truly montane species in Central Europe, given its absence in lowland and alpine lakes.

Растительность акватории и побережий озера Таловское и его окрестностей (Корякский округ, Камчатский край)

Растительность акватории и побережий озера Таловское и его окрестностей (Корякский округ, Камчатский край)

Contrasting Patterns in Chironomid (Chironomidae) Communities of Shallow and Deep Boreal Lakes Since the 1960s

Chironomids have been widely used as indicators of trophic conditions of lakes due to their species-specific environmental requirements. In order to understand resilience and deterioration of aquatic ecosystems due to increases and/or decreases in external loading, recent chironomid community and water chemistry data of seven southern Finnish lakes with varying anthropogenic pressures and bathymetric properties were compared with similar data from the 1960s. Altogether 64 taxa were found. At present, the most numerous taxa are Procladius spp., Chironomus f.l. plumosus, Chironomus f.l. salinarius, Stictochironomus f.l. psammophilus, Benthalia spp., Tanytarsus spp. and Cladotanytarsus. In four lakes, current abundances of chironomid larvae are clearly lower, and in two lakes abundances are higher than during the 1960s. In two shallow, previously highly eutrophic lakes, the chironomid community composition revealed an improvement in the trophic status, while in three deep, previously moderately oligotrophic lakes, a slight deterioration was found. The most dramatic increase in chironomid density and diversity was found in a lake which was most heavily polluted in 1960s. In deep lakes, a slow gradual eutrophication seemed to be responsible for the observed replacement of the previous oligotrophic communities, dominated by Monodiamesa bathyphila, Stictochironomus spp., Polypedilum f.l. breviantennatum and Heterotanytarsus apicalis by eutrophic taxa such as Chironomus f.l. salinarius, Chironomus f.l. plumosus, Einfeldia spp., Sergentia coracina and Microtendipes spp. Among the key factors responsible for changes were dissolved oxygen concentration, food availability, sediment quality, and in one lake, toxic effluents from industry.

The response of simulated grassland communities to the cessation of grazing

Changes in land-use are supposed to be among the severest prospective threats to plant diversity world-wide. In semi-natural temperate grasslands, the cessation of traditional land use like livestock grazing is considered to be one of the most important drivers of the diversity loss witnessed within the last decades. Despite of the enormous number of studies on successional pathways following grazing abandonment there is no general pattern of how grassland communities are affected in terms of diversity, trait composition and pace of succession. To gain a comprehensive picture is difficult given the heterogeneity of environments and the time and effort needed for long-term investigations. We here use a proven individual- and trait-based grassland community model to analyze short- and long-term consequences of grazing abandonment under different assumptions of resource availability, pre-abandonment grazing intensity and regional isolation of communities.Grazing abandonment led to a decrease of plant functional type (PFT) diversity in all but two scenarios in the long-term. In short-term we also found an increase or no change in Shannon diversity for several scenarios. With grazing abandonment we overall found an increase in maximum plant mass, clonal integration and longer lateral spread, a decrease in rosette plant types and in stress tolerant plants, as well as an increase in grazing tolerant and a decrease in grazing avoiding plant types. Observed changes were highly dependent on the regional configuration of communities, prevalent resource conditions and land use intensity before abandonment. While long-term changes took around 10–20 years in resource rich conditions, new equilibria established in resource poor conditions only after 30–40 years.Our results confirm the potential threats caused by recent land-use changes and the assumption that oligotrophic communities are more resistant than mesotrophic communities also for long-term abandonment. Moreover, results revealed that species-rich systems are not per se more resistant than species-poor grasslands.

The influence of different environmental conditions upon the initial development and ecological dynamics of phytobenthic communities

The combined influences of three environmental factors (nutrients, current velocity and grazers) upon the early establishment of stream phytobenthic communities (initial 5 weeks) were investigated in North West England. Nutrients were found to be more influential upon communities than current velocity or grazing. In eutrophic conditions, growth rates and diversity were greater and community dynamics appeared to be ultimately controlled by the biotic process of inter-specific competition. In oligotrophic conditions, although there was a large colonization pool, the communities were dominated by two species. These species were most likely selected for their ability to efficiently assimilate nutrients which supported the concepts postulated by the harsh-benign hypothesis. Furthermore, the oligotrophic communities produced a higher total biovolume than the more diverse eutrophic communities. The increased amount of species turnover in the eutrophic, compared to the oligotrophic systems, may represent community stability, suggesting support for the diversity-stability concept

К синтаксономии мелкобугристых болотных комплексов в лесотундре и тундре на севере Кольского полуострова

Mires of the Kola Peninsula were investigated mainly in its southern part (Botch, 1989; Payanskaya-Gvozdeva, 1990; Smagin, 1999а, 1999b). The published relevés of tundra mires of the Peninsula are poor (Kalliola, 1939; Koroleva, 2001, 2006), and this paper surveys pounikkos mire complexes in its northern part, represents their classification according to the Braun-Blanquet approach accompanied by syntaxonomical tables. The investigated area is situated in the north of the Kola Peninsula within the forest tundra and in southern arctic tundra, with the mean annual air temperature between –0.5 °C and –2.0 °C and annual precipitation 600–700 mm. The geological basis is Archean and Proterozoic crystalline rocks, covered by coarse gravelly moraine and sandy fluvial-glacial deposits, and peat deposits in wetlands. Plant cover represents the combination of subarctic birch forests, tundra and mires. The mire complexes occur in river valleys, in lake and between moraine hill depressions, alternating with shrub and dwarf-shrub lichen tundra and forest-tundra on hill tops and slopes. Peat hummock (pounikkos) height is 0.3–0.5 (up to 1.0) м, their diameter is 0.3–0.7 (up to 1.5) м. In July the ice core is kept at 20–30 cm below the surface. Water-logged narrow ditches separate hummocks from each other. Inundated sedges- and water-cotton-dominated lawns, willow thickets along the water currents and small lakes are common within the mire complex. In the 2012 field season mounds and hummocks were densely inhabited by rodents, in particular by Lemmus lemmus. Oligotrophic and ombrogenous mosses lichens dwarf-shrub dominated communities of mounds, small turf hummocks (pounus, pounikkos) and on the margin of mire massifs belong to the alliance Oxycocco−Empetrion hermaphroditi Nordh. 1936 ex Neuhäusl 1969 (diagnostic taxa (DT): Eriophorum vaginatum, Oxycoccus microcarpus, Rubus chamaemorus, Mylia anomala, Sphagnum fuscum) with the ass. Empetro−Sphagnetum fusci subass. typicum Dierssen (1982) 1996 (DT Ledum palustre and Orthocaulis kunzeanus) and Empetro−Sphagnetum fusci subass. dicranetosum elongati (Regel 1923) stat. nov. (DT Dicranum elongatum). Species poor sedges- and cottongrass dominated oligo-, oligomesotrophic fens of flark and lawns belong to the alliance Caricion rotundatae (Kalliola 1939) stat. nov. (DT Carex concolor, C. rariflora, C. rotundata, Baeothryon cespitosum, Eriophorum polystachion, Sphagnum lindbergii, S. compactum, Warnstorfia exannulata, Sarmentypnum sarmentosum, Calliergon stramineum) with three associations: ass. Gymnocoleo inflatae−Caricetum rariflorae ass. nov. (DT Nardus stricta, Sphagnum compactum, Gymnocolea inflata, Anthelia juratzkana), ass. Warnstorfio sarmentosi−Caricetum concoloris ass. nov. (DT the same as for the alliance), ass. Eriophoro polystachii−Sphagnetum lindbergii (Kalliola 1939) stat. nov. (DT Sphagnum lindbergii). Low willows and sedges dominated communities along mire springs or in percolated areas are described as ass. Salici lapponi−Caricetum concoloris ass. nov. (DT Betula nana, Salix lapponum, S. phylicifolia, Calamagrostis strictа, Carex rostrata, Comarum palustre, Molinia caerulea, Viola palustris, Lophozia longiflora) with no affiliation to any alliance. Proposed new alliance Caricion rotundatae occurs in tundra and forest-tundra zone to the north of main areal of alliances Rhynchosporion albae Koch 1926 and Caricion lasiocarpae Vanden Berghen ap. Lebrun et al. 1949, which comprise oligotrophic and mesotrophic flark communities of boreal mires. Among diagnostic taxa of Caricion rotundatae are low sedges and brown mosses and in opposite to the alliance Rhynchosporion albae it doesn’t include the character species of Rhynchosporion albae (Carex livida, Scheuchzeria palustris, Rhynchospora alba). The studied mire complexes are very characteristic for the entire subarctic Fennoscandia, being located in the north of the Kola Peninsula close to the east-northern border of their European distribution area. They are considered to include more fast-growing permafrost hummocks comparing with palsa mires (Grab, 2003). Based upon their high alpha diversity and important ecological and scientific role, the discussed pounus mire massifs are perspective to be examined as possible value natural habitats.

Ecological responses to climate change in a bird-impacted High Arctic pond (Nordaustlandet, Svalbard)

A 28-cm sediment core from an Arctic pond (Nordaustlandet, Svalbard), which is currently subjected to the fertilizing effect of bird guano, was analysed for fossil invertebrates and the physical properties of the sediment. The objective was to examine aquatic community responses to climate warming. Our record reveals that faunal changes have occurred. Initially chironomid assemblages were dominated by a cold-indicating oligotrophic community but this was replaced by a community typical of more nutrient-enriched conditions and warmer water temperature at around AD 1,700–1,800. After AD 1,800, ostracods and Daphnia increase suggesting that a nutrient enrichment threshold was crossed, probably related to increased planktonic algal productivity. In the early twentieth century, organic content markedly increases and magnetic susceptibility values suddenly drop, indicating a further increase in nutrient input and lake productivity. Since the most likely source of nutrients in the lake is goose guano, this suggests that the size of the bird colony may also have increased over this period. These changes coincide with climate warming suggesting a positive feedback in which climate change is the primary driver of the increasing geese abundance and lake productivity. Our results further suggest that the predicted future warming in the Arctic will continue to have cascading effects on freshwater ecosystems in the region.

Outbreaks by canopy-feeding geometrid moth cause state-dependent shifts in understorey plant communities.

The increased spread of insect outbreaks is among the most severe impacts of climate warming predicted for northern boreal forest ecosystems. Compound disturbances by insect herbivores can cause sharp transitions between vegetation states with implications for ecosystem productivity and climate feedbacks. By analysing vegetation plots prior to and immediately after a severe and widespread outbreak by geometrid moths in the birch forest-tundra ecotone, we document a shift in forest understorey community composition in response to the moth outbreak. Prior to the moth outbreak, the plots divided into two oligotrophic and one eutrophic plant community. The moth outbreak caused a vegetation state shift in the two oligotrophic communities, but only minor changes in the eutrophic community. In the spatially most widespread communities, oligotrophic dwarf shrub birch forest, dominance by the allelopathic dwarf shrub Empetrum nigrum ssp. hermaphroditum, was effectively broken and replaced by a community dominated by the graminoid Avenella flexuosa, in a manner qualitatively similar to the effect of wild fires in E. nigrum communities in coniferous boreal forest further south. As dominance by E. nigrum is associated with retrogressive succession the observed vegetation state shift has widespread implications for ecosystem productivity on a regional scale. Our findings reveal that the impact of moth outbreaks on the northern boreal birch forest system is highly initial-state dependent, and that the widespread oligotrophic communities have a low resistance to such disturbances. This provides a case for the notion that climate impacts on arctic and northern boreal vegetation may take place most abruptly when conveyed by changed dynamics of irruptive herbivores.Electronic supplementary materialThe online version of this article (doi:10.1007/s00442-013-2648-1) contains supplementary material, which is available to authorized users.

Changes in zooplankton community structure associated with the disappearance of invasive alewife in Saginaw Bay, Lake Huron

We evaluated the response of the zooplankton community Saginaw Bay, Lake Huron to the disappearance of the planktivore alewife Alosa pseudoharengus using data collected in 1991–1996 (pre alewife decline) and 2009–2010 (post alewife decline). Bosmina longirostris, Diaptomidae, Cyclops, and Daphnia galeata contributed greatly to the separation of the two time periods with Diaptomidae and D. galeata increasing and Cyclops and B. longirostris decreasing, although B. longirostris remained the dominant species. Peak densities of zooplankton occurred in early summer (June) in the 1990s and in early fall (October) in 2009–2010. For the analysis of environmental variables on a bay-wide, annual basis, abundance of alewife, age-0 yellow perch Perca flavescens and Bythotrephes captured much of the variation in annual zooplankton community structure. Abundances of Bythotrephes and age-0 yellow perch were both higher in 2009–2010 than in 1991–1996. Some changes such as increasing proportions of calanoid copepods reflect a more oligotrophic community and are potentially indicative of resource-driven changes rather than direct or indirect impacts of the alewife disappearance.

Ecological-physiological diversity of microbial communities in the anthropogenically impacted landscapes of Kuzbass

Data on the viable pool of microorganisms detected in the upper layer of technogenic soils formed on coal dumps, ash dumps and ore slurries are presented. The ecophysiological features of oligotrophic bacteria communities have been revealed; tiny colonial forms and biofilms of prokaryotes have been detected; the effect of recultivation and soil improvers on the composition of the major groups of microorganisms has been shown.

Cell size dependence of additive versus synergetic effects of UV radiation and PAHs on oceanic phytoplankton

Polycyclic Aromatic Hydrocarbons’ (PAHs) toxicity is enhanced by the presence of ultraviolet radiation (UVR), which levels have arisen due to the thinning of the ozone layer. In this study, PAHs’ phototoxicity for natural marine phytoplankton was tested. Different concentrations of a mixture of 16 PAHs were added to natural phytoplankton communities from the Mediterranean Sea, Atlantic, Arctic and Southern Oceans and exposed to natural sunlight received in situ, including treatments where the UVR bands were removed. PAHs’ toxicity was observed for all the phytoplankton groups studied in all the waters and treatments tested, but only for the pico-sized group a synergetic effect of the mixture and UVR was observed ( p = 0.009). When comparing phototoxicity in phytoplankton from oligotrophic and eutrophic waters, synergy was only observed at the oligotrophic communities ( p = 0.02) where pico-sized phytoplankton dominated. The degree of sensitivity was related to the trophic degree, decreasing as Chlorophyll a concentration increased.

Response of heterotrophic and autotrophic microbial plankton to inorganic and organic inputs along a latitudinal transect in the Atlantic Ocean

Abstract. The effects of inorganic and/or organic nutrient inputs on phytoplankton and heterotrophic bacteria have never been concurrently assessed in open ocean oligotrophic communities over a wide spatial gradient. We studied the effects of potentially limiting inorganic (nitrate, ammonium, phosphate, silica) and organic nutrient (glucose, aminoacids) inputs added separately as well as jointly, on microbial plankton biomass, community structure and metabolism in five microcosm experiments conducted along a latitudinal transect in the Atlantic Ocean (from 26° N to 29° S). Primary production rates increased up to 1.8-fold. Bacterial respiration and microbial community respiration increased up to 14.3 and 12.7-fold respectively. Bacterial production and bacterial growth efficiency increased up to 58.8-fold and 2.5-fold respectively. The largest increases were measured after mixed inorganic-organic nutrients additions. Changes in microbial plankton biomass were small as compared with those in metabolic rates. A north to south increase in the response of heterotrophic bacteria was observed, which could be related to a latitudinal gradient in phosphorus availability. Our results suggest that organic matter inputs will result in a predominantly heterotrophic versus autotrophic response and in increases in bacterial growth efficiency, particularly in the southern hemisphere. Subtle differences in the initial environmental and biological conditions are likely to result in differential microbial responses to inorganic and organic matter inputs.

Plant communities dominated by Pinus mugo agg. in Central Europe – comparison of the oligotrophic communities rich in Sphagnum

Plant communities dominated by Pinus mugo agg. in Central Europe – comparison of the oligotrophic communities rich in Sphagnum

Impact of light on marine bacterioplankton community structure

Reports of widespread proteorhodopsin and bacteriochlorophyll a genes suggest that light may facilitate the growth of many marine bacteria. We tested this hypothesis by conducting 3 light manipulation experiments with bacterial communities obtained off the Southern California coast. In each experiment, duplicate 20 l mesocosms were subjected to either 14:10 h light:dark or continual darkness for 5 to 10 d. Automated Ribosomal Intergenic Spacer Analysis (ARISA), a whole- community fingerprinting technique, was used to determine how light affects bacterial community structure. Light removal resulted in only minor changes in ARISA profiles (mean decrease in similar- ity between treatments: 6 ± 1% via Sorenson's index (only considers presence/absence of taxa), 15 ± 7% via Pearson's coefficient (considers relative abundance of taxa)) at the conclusion of the experi- ments. Oligotrophic communities responded nearly twice as strongly to light removal compared to mesotrophic communities. Phototrophs such as cyanobacteria exhibited consistent, sharp declines in dark treatments, indicating whatever heterotrophic abilities they possess were not sufficient to sus- tain them when faced with natural removal processes such as grazing or viral lysis. Members of the broad SAR86, SAR11, SAR116, CFB and Roseobacter phylogenetic groups exhibited minor, mixed responses to light removal, suggesting that only a few select members may rely on phototrophy to a measurable extent. Our results indicate that the majority of non-cyanobacterial bacteria in the ocean do not depend heavily on light to maintain themselves over 5 or 10 d periods in the presence of nat- ural removal processes, and hint that proteorhodopsin and bacteriochlorophyll a genes may have alternative, undetermined ecological benefits besides phototrophy.

A hypothesis for the assessment of the importance of microbial food web linkages in nearshore and offshore habitats of the Laurentian Great Lakes

Our work in the Laurentian Great Lakes of North America indicates that significant fluxes of carbon and phosphorus can pass through the microbial food webs (MFW) of these lakes. Here we present a synthesis of our recent investigations conducted largely along a trophic axis from the heavily eutrophic coastal Sandusky Bay to offshore communities near the international boundary in the central basin of Lake Erie. We find that the significance of the MFW in transporting C and P to higher trophic levels differs along a trophic gradient. In relatively eutrophic nearshore communities, most C and P are fixed into phytoplankton, transport of materials is largely dependent on grazing by cladocerans, and transport through the MFW is relatively insignificant. In contrast, in relatively oligotrophic offshore communities bacterial biomass often exceeds phytoplankton biomass, the majority of P is fixed into bacteria, bacterivorous grazers (e.g. rotifers and protozoa) dominate, copepods are the dominant microcrustacean, and transport of C and P through the MFW represents a major pathway. We suggest that the management of large-lake ecosystems is largely based on relatively eutrophic “nearshore” views of the base of the food web and needs to be modified to include considerations of the MFW in the more oligotrophic offshore regions of these lakes.