Resistance and resilience of benthic biofilm communities from a temperate saltmarsh to desiccation and rewetting

Periods of desiccation and rewetting are regular, yet stressful events encountered by saltmarsh microbial communities. To examine the resistance and resilience of microbial biofilms to such stresses, sediments from saltmarsh creeks were allowed to desiccate for 23 days, followed by rewetting for 4 days, whereas control sediments were maintained under a natural tidal cycle. In the top 2 mm of the dry sediments, salinity increased steadily from 36 to 231 over 23 days, and returned to seawater salinity on rewetting. After 3 days, desiccated sediments had a lower chlorophyll a (Chl a) fluorescence signal as benthic diatoms ceased to migrate to the surface, with a recovery in cell migration and Chl a fluorescence on rewetting. Extracellular β-glucosidase and aminopeptidase activities decreased within the first week of drying, but increased sharply on rewetting. The bacterial community in the desiccating sediment changed significantly from the controls after 14 days of desiccation (salinity 144). Rewetting did not cause a return to the original community composition, but led to a further change. Pyrosequencing analysis of 16S rRNA genes amplified from the sediment revealed diverse microbial responses, for example desiccation enabled haloversatile Marinobacter species to increase their relative abundance, and thus take advantage of rewetting to grow rapidly and dominate the community. A temporal sequence of effects of desiccation and rewetting were thus observed, but the most notable feature was the overall resistance and resilience of the microbial community.     

The relative importance of light and nutrient limitation of phytoplankton growth: a simple index of coastal ecosystem sensitivity to nutrient enrichment

Anthropogenic nutrient enrichment of the coastal zone is now a well-established fact. However, there is still uncertainty about the mechanisms through which nutrient enrichment can disrupt biological communities and ecosystem processes in the coastal zone. For example, while some estuaries exhibit classic symptoms of acute eutrophication, including enhanced production of algal biomass, other nutrient-rich estuaries maintain low algal biomass and primary production. This implies that large differences exist among coastal ecosystems in the rates and patterns of nutrient assimilation and cycling. Part of this variability comes from differences among ecosystems in the other resource that can limit algal growth and production – the light energy required for photosynthesis. Complete understanding of the eutrophication process requires consideration of the interacting effects of light and nutrients, including the role of light availability as a regulator of the expression of eutrophication. A simple index of the relative strength of light and nutrient limitation of algal growth can be derived from models that describe growth rate as a function of these resources. This index can then be used as one diagnostic to classify the sensitivity of coastal ecosystems to the harmful effects of eutrophication. Here I illustrate the application of this diagnostic with light and nutrient measurements made in three California estuaries and two Dutch estuaries.     

Modification of benthic insect communities in polluted streams: combined effects of sedimentation and nutrient enrichment

Responses of the benthic insect community of a southern Appalachian trout stream to inorganic sedimentation and nutrient enrichment were monitored over a period of eight months. Entry of pollutants from point sources established differentially polluted zones, allowing an assessment of impacts due to sedimentation alone and in association with elevated nutrient levels. Input of sediment resulted in a significant increase in bed load and decrease of pH at the substrate-water interface (P < 0.05). The zone receiving nutrient runoff from livestock pasture exhibited elevated levels of nitrate and phosphate, but available data indicated such concentrations to be quite low. Species richness, diversity, and total biomass of filter feeding Trichoptera and Diptera, predaceous Plecoptera, and certain Ephemeroptera were significantly reduced in the polluted zones. Inorganic sedimentation, operating indirectly through disruption of feeding and filling of interstitial spaces, was considered to be the primary factor affecting filter feeding taxa. Decomposition of compounds associated with materials in the bed load may depress pH and eliminate acid sensitive species of Plecoptera and Ephemeroptera. Such processes of acidification may be particularly important to Appalachian streams since the pH of regional surface waters is characteristically acidic prior to sedimentation. Accumulation of particles on body surfaces and respiratory structures, perhaps as a function of wax and mucous secretion or surface electrical properties, appears to be the major direct effect of inorganic sedimentation on stream insects. Growths of the filamentous bacterium Sphaerotilus natans were also frequently associated with silted individuals in the zone receiving nutrient addition. Distribution of the bacterium suggested that silted substrates, perhaps as related to the presence of iron compounds, are required for colonization in dilute nutrient solutions. The primary effect of Sphaerotilus colonies appears to be augmentation of particle accumulation through net formation by bacterial filaments. Data indicate that inorganic sedimentation and nutrient addition operate synergistically, eliminating a significantly greater number of taxa than exposure to one pollutant alone.     

The response of two estuarine benthic communities to the quantity and quality of food

Experimental manipulations of food supply were performed on soft sediment cores from two European estuaries, the Westerscheldt and the Gironde, with a view to determining benthic macrofaunal community response. Over a period of twenty weeks in a laboratory mesocosm system, both communities showed losses in terms of numbers of individuals and small, but non-significant, losses in terms of numbers of species. Whereas no effect of the different types of foods or the dose levels at which they were supplied was detected for the Westerscheldt benthic community, that of the Gironde showed some significant response. This was largely attributed to the differential mortality of spionid polychaetes across the dose levels used, with the highest dose, equivalent to 200 g C m^–2 yr^–1, only just maintaining their initial population densities. The results are discussed in terms of the importance of lateral advection of food particles at the benthic boundary layer and the general insufficiency of many estimates of carbon input to shallow benthic systems.     

Estuarine intertidal sandflat benthic microalgal responses to in situ and mesocosm nitrogen additions

Benthic microalgal communities are important components of estuarine food webs and make substantial contributions to coastal materials cycling. Nitrogen is generally the limiting factor for marine primary production; however other factors can limit benthic primary producers because of their access to the additional nutrients found in sediment porewater. Field and laboratory experiments were conducted to test the hypothesis that water column nitrogen supply affects estuarine sandflat benthic microalgal community structure and function. Our field and mesocosm experiments assessed changes at both the population and functional group levels. Simulated water column nitrogen additions increased maximum community photosynthesis in most cases (Pb_max from photosynthesis vs. irradiance curves). Additional changes that resulted from nitrogen additions were decreases in porewater phosphate, increases in porewater ammonium, shifts in community composition from N₂ fixing cyanobacteria toward diatoms, and detectable, though not statistically significant increases in biomass (as chlorophyll a). Results from field and laboratory experiments were quite similar, suggesting that laboratory experiments support accurate predictions of the response of intertidal benthic microalgae to changes in water column nutrient conditions.     

Herbivore responses to nutrient enrichment and landscape heterogeneity in a mangrove ecosystem

Complex gradients in forest structure across the landscape of offshore mangrove islands in Belize are associated with nutrient deficiency and flooding. While nutrient availability can affect many ecological processes, here we investigate how N and P enrichment interact with forest structure in three distinct zones (fringe, transition, dwarf) to alter patterns of herbivory as a function of folivory, loss of yield, and tissue mining. The effects of nutrient addition and zone varied by functional feeding group or specific herbivore. Folivory ranged from 0 to 0.4% leaf area damaged per month, but rates did not vary by either nutrient enrichment or zone. Leaf lifetime damage ranged from 3 to 10% of the total leaf area and was caused primarily by the omnivorous tree crab Aratus pisonii. We detected two distinct spatial scales of response by A. pisonii that were unrelated to nutrient treatment, i.e., most feeding damage occurred in the fringe zone and crabs fed primarily on the oldest leaves in the canopy. Loss of yield caused by the bud moth Ecdytolopha sp. varied by zone but not by nutrient treatment. A periderm-mining Marmara sp. responded positively to nutrient enrichment and closely mirrored the growth response by Rhizophora mangle across the tree height gradient. In contrast, a leaf-mining Marmara sp. was controlled by parasitoids and predators that killed >89% of its larvae. Thus, nutrient availability altered patterns of herbivory of some but not all mangrove herbivores. These findings support the hypothesis that landscape heterogeneity of the biotic and abiotic environment has species-specific effects on community structure and trophic interactions. Predicting how herbivores respond to nutrient over-enrichment in mangrove ecosystems also requires an assessment of habitat heterogeneity coupled with feeding strategies and species-specific behavior measured on multiple scales of response.     

Immigration and zooplankton community responses to nutrient enrichment: a mesocosm experiment

Dispersal can be an important determinant of local diversity and species composition, but evidence for effects of the regional species pool on local zooplankton communities has been mixed. Theory and experiments suggest that immigration will be necessary for maintenance of community diversity and functioning during periods of environmental change; conversely, fluctuating resource levels may increase the likelihood of invasion success. We conducted a factorial-design mesocosm experiment to test the effects of a nutrient pulse and weekly immigration from other lakes on the diversity and composition of a pelagic zooplankton community. Contrary to expectations, there were no interactive effects of nutrient enrichment and immigration on any measure of diversity, and the initial shift in community composition in response to the nutrient pulse did not depend on the introduction of new species or genotypes from more productive lakes. Although immigration increased species richness in enclosures, success of most colonising species was poor. However, the dispersal treatment appears to have enabled a stronger predator response to increased herbivore numbers in nutrient-pulsed enclosures, leading to an eventual decline in the abundance of some herbivorous species in response to immigration. We conclude that community invasibility was not influenced by productivity, and that dispersal limitation did not strongly constrain the response of the zooplankton community to our applied disturbance. This indicates an unexpected resistance to change in species composition and diversity in spite of disturbance, and suggests that, in our study system, changes in the abundance of resident species are more important than introductions of new species in the community response to short-term environmental change.Electronic Supplementary Material Supplementary material is available to authorized users in the online version of this article at .     

Benthic community responses to nutrient enrichment and predator exclusion: Influence of background nutrient concentrations and interactive effects

Potential community effects of nutrient enhancement are a topic of theoretical interest and increasing management concern in coastal marine systems. While increased nutrient levels may lead to increased microalgal production and biomass, studies have provided variable evidence regarding the existence of upward cascade effects on macrofauna. In benthic marine communities, limitation by predation or factors preventing recruitment response may contribute to weak coupling between resource availability and macrobenthos abundances. We conducted blocked nutrient addition and predator exclusion experiments in the intertidal of two estuaries that varied in background nutrient concentrations (Cape Fear and White Oak, southeastern North Carolina). Benthic community comparisons were also made among these and two other North Carolina estuaries to examine correlations in distribution patterns. Cape Fear, which had the highest background nitrogen and phosphorus concentrations, also had highest ambient benthic microalgal biomass. There was no significant response of microalgal biomass to local nutrient additions in Cape Fear and only one macrofaunal taxon during one season exhibited abundance responses to nutrient additions. White Oak, with lower background nutrient levels, was characterized by significant microalgal responses to nutrient additions and significant macrofauna abundance responses for 50% of the species examined during summer experiments. However, all of these macrofauna declined in abundance with nutrient enhancement while biomass remained constant or significantly increased with nutrient additions. This suggests a complex response of macrofauna to nutrient additions in this estuary with greater biomass per individual but a corresponding decline in abundances. Top-down/bottom-up interactive effects were observed for haustoriid amphipods, which were uncommon or absent when predators had access, but exhibited strong biomass responses to nutrient enhancement when predators were excluded. These results support a growing body of literature that indicates the importance of background conditions in regulating benthic community responses to nutrient enhancement. However, responses may be complex with biomass per individual rather than densities being the primary response variable for some taxa and predator moderation of responses occurring for some taxa but not others.     

Changes in stoichiometric constraints on epilithon and benthic macroinvertebrates in response to slight nutrient enrichment of mountain rivers

1. To assess changes in stoichiometric constraints on stream benthos, we measured elemental composition of epilithon and benthic macroinvertebrates in intrinsically P‐limited mountain rivers, upstream and downstream of low‐level anthropogenic nutrient enrichment by effluents of municipal wastewater treatment plants. 2. While there was a broad range in the elemental composition of epilithon (C : P ratios of 200–16 500, C : N ratios of 8–280, N : P ratios of 8–535) and heptageniid mayfly scrapers (C : P ratios of 125–300, C : N ratios of 5.1–7.2, N : P ratios of 20–60), the average C : P ratio of epilithon was 10‐fold lower and the average C : N ratio twofold lower at more nutrient‐rich downstream sites. Nutrient ratios in benthic macroinvertebrates were lower than in epilithon and varied little between relatively nutrient‐poor and nutrient‐rich sites. 3. We modified the existing definition of producer‐consumer elemental imbalance to allow for variation in consumer nutrient content. We defined this ‘non‐homeostatic’ imbalance as the perpendicular distance between the producer and consumer C : P, C : N, or N : P ratios, and the 1 : 1 line. 4. At P‐limited sites, the estimated mayfly N : P recycling ratio was higher than the N : P ratio in epilithon, suggesting nutrient recycling by consumers could accentuate P‐limitation of epilithon. 5. Measuring the degree of producer–consumer nutrient imbalance may be important in predicting the magnitude of effects from nutrient enrichment and can help elucidate the causes and consequences of ecological patterns and processes in rivers.     

Growth and reproductive responses of Laminaria longicruris (Laminariales,Phaeophyta) to nutrient enrichment

A series of comparative culture experiments were conducted in order to determine responses of Laminaria longicruris male and female gametophytes and juvenile sporophytes to several temperatures (5, 10, 15, 20 °C), light levels (10, 35, 75 µmol m^–2 s^–1) and media nitrogen concentrations (0, 20, 100 µM ammonium-nitrogen). Responses were measured as numbers of male and female gametophytes producing gametangia and number of sporophytes produced following fertilization. Both male and female gametogenesis was reduced at 5 and 20 °C versus 10 and 15 °C. At 20 °C gametogenesis inhibition was greater with higher levels of ammonium-nitrogen concentration (100 µM). Sporophyte production was more sensitive to light, temperature and nitrogen concentration than gametogenesis. Production of sporophytes was inhibited completely at 20 °C. At lower temperatures, increasingly higher nutrient concentrations produced greater inhibition of production of sporophytes.     

Spatial heterogeneity in soil nutrient supply modulates nutrient and biomass responses to multiple global change drivers in model grassland communities

Changes in the atmospheric concentration of carbon dioxide ([CO₂]), nutrient availability and biotic diversity are three major drivers of the ongoing global change impacting terrestrial ecosystems worldwide. While it is well established that soil nutrient heterogeneity exerts a strong influence on the development of plant individuals and communities, it is virtually unknown how nutrient heterogeneity and global change drivers interact to affect plant performance and ecosystem functioning. We conducted a microcosm experiment to evaluate the effect of simultaneous changes in [CO₂], nutrient heterogeneity (NH), nutrient availability (NA) and species evenness on the biomass and nutrient uptake patterns of assemblages formed by Lolium perenne, Plantago lanceolata and Holcus lanatus. When the nutrients were heterogeneously supplied, assemblages exhibited precise root foraging patterns, and had higher above‐ and belowground biomass (average increases of 32% and 29% for above‐ and belowground biomass, respectively). Nutrient heterogeneity also modulated the effects of NA on biomass production, complementarity in nitrogen uptake and below: aboveground ratio, as well as those of [CO₂] on the nutrient use efficiency at the assemblage level. Our results show that nutrient heterogeneity has the potential to influence the response of plant assemblages to simultaneous changes in [CO₂], nutrient availability and biotic diversity, and suggest that it is an important environmental factor to interpret and assess plant assemblage responses to global change.     

The effects of artificial enrichment upon the planktonic and benthic communities in a mesotrophic to hypertrophic loch series in lowland Scotland

Phytoplankton, submerged macrophytes, zoobenthos and zooplankton communities were studied concurrently in three lochs which progressively increased in trophic state. The phytoplankton showed a change in dominance from Bacillariophyceae (Pennales) and Desmidaceae in the mesotrophic Loch of the Lowes to Bacillariophyceae (Pennales) and Cyanobacteria in eutrophic Balgavies Loch and to Bacillariophyceae (Centrales) and Chlorophyceae in hypertrophic Forfar Loch. The Nygaard Compound phytoplankton quotient rose from 1.2 in the Lowes to 12 in Forfar. There was a decrease in number of species of submerged macrophytes, and in depth of colonisation from 5 m in the Lowes to 2 m in Forfar which was proportional to the decrease in mean transparency. Macrophyte biomass, however, increased in proportion to the concentration of total phosphorus in the lochs. There was a decrease in species of littoral zoobenthos, but an increase in density of profundal benthos from the Lowes to Forfar. Zooplankton also showed an increase in density with enrichment, accompanied by changes in dominance. Calanoida were dominant in the Lowes; Cladocera in Balgavies and Cyclopoda in Forfar. The importance of these differences in understanding the effects of progressive enrichment and to the management of the lochs is discussed.     

Short-term responses of phytoplankton to nutrient enrichment and planktivorous fish predation in a temperate South American mesotrophic reservoir

The benthic macroinvertebrate community is an important component of stream diversity, because its members are fundamental connectors among the different trophic levels of running waters. In this study, we assessed alpha and beta diversities of benthic macroinvertebrates in three stream sites and four microhabitats: (i) moss in the air-water interface; (ii) submerged roots of terrestrial plants; (iii) leaf litter deposited in pools; (iv) stones in riffles. We constructed rarefaction curves and compared species richness among microhabitats for each stream site. Additionally, we evaluated which factor, stream site, or microhabitat, was most important in determining variation in assemblage structure, i.e., beta diversity. There was no significant difference among microhabitats in terms of taxa richness evaluated by rarefaction curves. Using partial Constrained Correspondence Analysis (pCCA), we found that microhabitat was most important in determining community composition, accounting for 42.02% of the total variation. Stream sites accounted for 22.27%. In accordance with the pCCA, exploratory multivariate methods (ordination and classification) revealed four distinct groups, corresponding to the four microhabitats, independent of stream sites. Our results indicated that differences among environmental conditions are much more important in the determination of stream assemblage structure than are differences in spatial location. Accordingly, adjacent microhabitats in a single stream site harbor macroinvertebrate assemblages more dissimilar than those found in a single microhabitat at different stream sites. Handling editor: D. Dudgeon     

Regional responses within the kidney to ischemia: assessment of adenine nucleotide and catabolite profiles

Renal cortex (C) has predominantly aerobic metabolism, whereas inner medulla (IM) has both aerobic and anaerobic capacities. This study was undertaken (1) to assess how well rat IM anaerobic metabolism maintains this region's ATP content during ischemia; and (2) to determine whether regional variations in adenylate pool/catabolite responses to ischemia exist, obscuring interpretation of cellular energetics in rat studies of acute renal failure (ARF). Adenine nucleotides/catabolites were measured in rat C, IM and outer medulla (OM) after 15 and 45 min of ischemia. After 15 min, all regions showed profound ATP depletion, although the IM maintained slightly higher (by 0.23 μmol/g) absolute ATP levels than C/OM tissues (normal ATP value = 8.7 μmol/g). By 45 min, significant differences in regional ATP levels did not exist. Striking regional catabolite differences were apparent at both 15 and 45 min. Most prominent were: (1) intrarenal purine base/inosine gradients, levels falling approx. 22–50% from C to IM; and (2) preferential OM AMP/IMP/adenosine accumulation. To assess whether more homogeneous results might be found in rabbit kidney, possibly making this animal preferable to rats for studies of renal ischemia, rabbit C, OM and IM adenylate pools were analyzed after 15 min of ischemia. C vs. IM ATP differences were greater (approx. 1.3 μmol/g) and large catabolite concentration differences were still apparent. Conclusions: (1) anaerobic mechanisms support IM ATP levels during ischemia but, in terms of normal concentrations, the impact is small, particularly in the rat; and (2) marked regional differences in adenylate catabolite levels exist within ischemic kidneys. These need to be recognized when analyzing adenylate pool responses in ischemic ARF.