Mechanisms regulating abundance of submerged vegetation in shallow eutrophic lakes

Shallow eutrophic lakes tend to be either in a turbid state dominated by phytoplankton or in a clear-water state dominated by submerged macrovegetation. Recent studies suggest that the low water turbidity in the clear-water state is maintained through direct and indirect effects of the submerged vegetation. This study examined what mechanisms may cause a recession of the submerged vegetation in the clear-water state, and thereby a switch to the turbid state. The spatial distribution of submerged vegetation biomass was investigated in two shallow eutrophic lakes in the clear-water state in southern Sweden. Biomass of submerged vegetation was positively correlated with water depth and wave exposure, which also were mutually correlated, suggesting that mechanisms hampering submerged vegetation were strongest at shallow and/or sheltered locations. The growth of Myriophyllum spicatum, planted in the same substrate and at the same water depth, was compared between sheltered and wave exposed sites in two lakes. After 6 weeks the plants were significantly smaller at the sheltered sites, where periphyton production was about 5 times higher than at the exposed sites. Exclosure experiments were conducted to evaluate the effects of waterfowl grazing on macrophyte biomass. Potamogeton pectinatus growth was decreased by grazing, whereas M. spicatum was not affected. The effects were greater at a sheltered than at a wave-exposed site, and also negatively related to distance from the reed belt. These results suggest that competition from epiphytes and waterfowl grazing hamper the development of submerged vegetation at sheltered and/or shallow locations. An increased strength of these mechanisms may cause a recession of submerged vegetation in shallow eutrophic lakes in the clear-water state and thereby a switch to the turbid state. Received: 24 June 1996 / Accepted: 8 September 1996     

Herbivory and intraspecific competition in a stream caddisfly population

Summary Field experiments were conducted to assess the effects of different population densities of the herbivorous caddisfly Helicopsyche borealis on periphyton biomass and on its own growth rate in Big Sulphur Creek, a third-order stream in northern California, USA. Stream enclosures were used to vary grazer density from one-eighth to twice natural density (1/8X–2X) in two experiments (35 d and 60 d), which spanned the period of most rapid larval growth. Periphyton biomass and chlorophyll a were inversely related to grazer density. Grazer densities of 1/8X–1/2X moderately reduced periphyton when compared to an ungrazed control, whereas densities of 1X–2X greatly depleted periphyton. The growth rate of H. borealis larvae declined with increasing larval density. Growth rate was highest at densities of 1/8X–1/2X; larvae grew more slowly at 1X and showed no growth at 2X. Weekly supplementation of periphyton to 1/4X, 1X, and 2X densities significantly increased the final body mass of larvae at 1X and 2X (by 26% and 50%, respectively, compared to unsupplemented larvae) but did not change the body mass of larvae at 1/4X. These results suggest that periphyton is a limiting resource in Big Sulphur Creek and that H. borealis larvae compete exploitatively for that resource. Intraspecific competition may be an important, but often overlooked, feature of many herbivore populations in streams.     

The feeding habits of a group of herbivorous rock-dwelling cichlid fishes (Cichlidae: Perciformes) from Lake Malawi,Africa

Synopsis The feeding; habits of a group of tropical herbivorous rock-dwelling cichlid fishes from Lake Malawi, Africa, are investigated using stomach content analyses. The various species fed selectively on the periphyton of the rocky shores. Blue-green alga of the genus Calothrix was the most common item ingested by the group. Diatoms (Chrysophyta) also were abundant food items. Discriminant analysis showed that dietary items were good variables to identify species. Interspecific dietary differences showed a continuum from those species feeding primarily on Calothrix to those feeding primarily on diatoms. Algal resources exhibit distinct patterns of spatial variation. Diet was correlated with foraging behavior and trophic morphology. Interspecific differences in diet could possibly facilitate ecological coexistence among various species. Such coexistence would contribute to the maintenance of the high diversity fish faunas characteristic of the Great Rift Lakes of Africa.     

Benthic freshwater cyanobacteria as indicators of anthropogenic pressures

Freshwater cyanobacteria are hardly used as biological indicators of anthropogenic pressures, possibly for two main reasons: (a) their response to anthropogenic pressures is often poorly known; (b) reliably identifying cyanobacteria species is a challenge for technicians and researchers. We assessed the usefulness of cyanobacteria species as biological indicators of two human stressors: the high orthophosphate input from urban wastewaters; the high nitrate concentration produced by agricultural land use. We analysed variation in benthic cyanobacterial assemblages at 85 sites in South-Central Spain as a response to eight environmental variables: pH, conductivity, temperature, altitude, nitrate, orthophosphate, irrigation land use and non-irrigation land use.Results revealed that conductivity was the main environmental factor that contributed to differences between assemblages. Orthophosphate was more influential for community composition than nitrate. Changes in species composition related to human pressures suggested that some cyanobacteria species (e.g. Nostoc verrucosum, Phormidium autumnale, Plectonema tomasinianum, Rivularia haematites, Tolypothrix distorta) could be useful tools for the bioindication of anthropogenic pressures; while others species provide more information about natural physicochemical reference conditions (Nostoc caeruleum, Phormidium fonticola). Further research into cyanobacteria and macroalgae assemblages in different impacted scenarios could help improve macrophyte indices.     

Analysis of structural and functional indicators for assessing the health state of mountain streams

Mountain streams play a key role in the conservation of aquatic biodiversity and key ecosystem services; however human activities are threatening these ecosystems as mountain areas become more and more developed and intensively used. Many of these streams are not considered in current national monitoring programs due to their small catchment area. However, assessing their status and monitoring their trends is well needed to ensure their proper management and conservation. In this study, we evaluated the use of a range of indicators related to different ecosystem structural and functional components in 2 streams affected by sewage outflows and compared with an unpolluted stream in the Picos de Europa National Park (Spain). We surveyed benthic periphyton, macroinvertebrate communities and fish assemblages and also estimated periphyton growth rates, wood decomposition rates and river metabolism. Additionally, we compared the performance of the selected indicators in different hydraulic conditions. Results revealed an effect of the organic pollution on most of the functional and structural indicators for the most polluted stream. Only the number of Ephemeroptera, Plecoptera and Trichoptera taxa, the Iberian Biomonitoring Working Party index, the invertebrate multimetric index used by the regional water agency, the fish abundance and biomass were sensitive enough to detect low levels of pollution and followed the expected response to the pollution degree. Moreover, most of the indicators behaved similarly under different hydraulic conditions, without major differences between pools and runs. However, the combination of both pool and run replicates at the reach scale resulted in a higher detection capacity of the effects of organic pollution.     

Periphyton as a potential phosphorus sink in the Everglades Nutrient Removal Project

Phosphorus uptake and release by periphyton mats were quantified in the Everglades Nutrient Removal Project (ENRP) to evaluate the potential for periphyton P removal. Short-term P uptake rates were determined by incubating cyanobacteria (Oscillatoria princeps and Shizothrix calcicola) and Chlorophycean (primarily Rhizoclonium spp.) algal mat samples for 0.5–2 h under ambient conditions in BOD bottles spiked with soluble reactive P (SRP). Cyanobacterial mats removed P more than twice as fast (80–164 μg P h⁻¹ g⁻¹ AFDM) as Chlorophycean mats (33–61 μg P h⁻¹ g⁻¹ AFDM) during these incubations. In a longer term study, fiberglass cylinders were used to enclose 1.8 m² plots within the wetland and were dosed weekly for 7 weeks with: (1) no nutrients; (2) SRP (0.25 g P m⁻² week⁻¹); or (3) SRP plus nitrate (0.42 g N m⁻² week⁻¹) and ammonium (0.83 g N m⁻² week⁻¹). Phosphorus uptake rates by this periphyton assemblage, which was dominated by the chlorophytes Stigeoclonium spp. and Oedogonium spp., were measured weekly and were similar among nutrient treatments on most dates, indicating that the algal storage compartment for P was not saturated despite repeated P additions. Decomposition rates and P loss by cyanobacteria and Chlorophycean mats were determined by measuring biomass loss and SRP release in darkened BOD bottles over 28–42 day periods under anaerobic and aerobic conditions. First-order aerobic and anaerobic decomposition rates for cyanobacterial mats (k = 0.1095 and 0.1408 day⁻¹, respectively) were 4–20-fold higher than rates for Chlorophycean mats (k = 0.0066 and 0.0250 day⁻¹, respectively) and cyanobacteria released considerably more P back to the water column. Our findings suggest that periphyton can be an important short-term sink for P in treatment wetlands and that retention is strongly affected by the taxonomic composition of the periphyton assemblage.     

三峡库区古夫河着生藻类分布与水质因子的关系

于2010年12月至2011年12月,对位于长江三峡库区内的湖北省兴山县古夫河着生的藻类和水体共采样10次,共鉴定出着生藻类28属,检测水质指标12项。对水质变量进行因子分析,根据特征值大于1的原则选取4个主成分,第一主成分主要代表总氮（TN）和硝酸盐氮（NO3--N）,第二主成分主要代表pH和化学需氧量（CODCr）,第三主成分主要代表磷酸盐磷（PO34--P）、总磷（TP）和氨氮（NH4＋-N）,第四主成分主要代表电导率（Cond）和总有机碳（TOC）。将提取的9项主要水质因子与着生藻类重要属进行典型对应分析（CCA）,结果可见,古夫河主要水质因子与着生藻类类群分布相关性大小为：CODCr〉pH〉TN〉NO3--N〉TOC〉TP〉PO34--P〉Cond〉NH4＋-N。Monte Carlo显著性检验表明,CODCr和pH对古夫河着生藻类物种组成及空间分布的影响最大,其次为TN、NO3--N和TOC。pH与CODCr呈极强的负相关关系,古夫河着生藻类大多数类群均与CODCr表现出正相关关系,仅胶毛藻属（Chaetophora）倾向低CODcr和偏碱性的环境。TN和NO3--N有很强正相关性,而两者均与TOC有强负相关,它们将古夫河着生藻类大体上分为两类,即亲碳元素的类群和亲氮元素的类群。TP通常是河流生态系统中的限制性营养因子,但在古夫河流域中并不是影响着生藻类物种分布的主要水质因子。     

Pine afforestation changes more strongly community structure than ecosystem functioning in grassland mountain streams

In the past decades, afforestation of grassland landscapes has gained importance both as an economic activity and a mechanism to mitigate anthropogenic carbon emissions. This study evaluates the effect of pine afforestation on grassland streams analyzing changes in two integrative ecological indicators: leaf litter breakdown and primary production. We compare those results with changes in structural attributes of benthic biota (primary producers and invertebrates). Six contiguous first-order streams were selected in the upper basin of the Ctalamochita river (Córdoba, Argentina): three reference streams draining grasslands and three streams draining Pinus elliottii afforestations. Two in situ experiments were performed to compare leaf litter breakdown and primary production between grassland and afforested streams. Additionally, invertebrate assemblages in leaf litter and riffles, and periphyton standing stock were sampled and assessed. Nine out of 26 structural indicators showed differences between stream types but indicators measuring changes at the basal level of the food web (i.e. detritus and primary producers) were less sensitive than those recording changes in consumers. Our attempt to measure direction and magnitude of changes on stream functioning following afforestation was halted by our simple implemented methodology (i.e. leaf pack method for leaf litter decay and biofilm accrual on natural stone substrates for primary production assessments); only 1 out of 4 indicators differed. We argue that the lack of strong differences in elemental measurements of primary production and needle decay between afforested and grassland streams resulted from compensating opposing forces controlling such processes, i.e. higher grazing vs. higher sunlight in grassland streams and higher shredding vs. lower microbial decomposition mediated by lower temperature in afforested streams. Attributes related to the invertebrate compartment showed the highest sensitivity to afforestation, emphasizing their value as biological indicators of stream ecological integrity.     

Distribution of egg strands of perch (<Emphasis Type="Italic">Perca</Emphasis> <Emphasis Type="Italic">fluviatilis</Emphasis> L.) with respect to depth and spawning substrate

Much of the historical Everglades has been either lost or degraded as a result of human activities. Among the aquatic habitats that comprise the Everglades landscape mosaic, open-water sloughs support critical ecological functions and appear especially sensitive to both hydrologic and water-quality perturbations. We used a combination of remote sensing and on-the-ground sampling to document spatial changes in the extent and vegetative composition of sloughs along a phosphorus (P) gradient in the northern Everglades. Increasing levels of water and soil P were associated with a decline in slough coverage, loss of the abundant native periphyton community, and a shift in dominant macrophyte species. The characteristic slough macrophyte species Eleocharis cellulosa and Nymphaea odorata exhibited different sensitivities to P enrichment, but both species declined with enrichment as slough habitats were invaded by Typha domingensis, a species that is known to expand aggressively in response to enrichment. A limited amount of open-water habitat occurred in highly enriched areas, but these habitats were maintained largely as a result of airboat disturbance and did not contain characteristic slough vegetation. Many changes in slough coverage and composition occurred in areas where water and soil P concentrations were only marginally higher than background levels. Our findings support the need for Everglades hydrologic restoration efforts to adhere to strict water-quality standards for P to avoid further degradation of this key landscape feature. Handling editor: L. M. Bini     

Spatial and seasonal variations in benthic algal assemblages in streams in monsoonal Hong Kong

Samples from stone surfaces were collected in pools within four unpolluted hillstreams (two shaded and two unshaded) in monsoonal Hong Kong (lat. 23°N) to elucidate the extent of spatial (within and among streams) and temporal (seasonal) variations in algal biomass and assemblage composition. Sampling continued for over 12 months, incorporating the dry season when streams were at baseflow, and the wet season when spates were frequent. We anticipated that algal biomass would be lower in shaded streams and during the wet season, with associated seasonal differences in assemblage composition or relative abundance of different growth forms (e.g. erect versus prostrate). Benthic chlorophyll a (a proxy for algal biomass) varied among streams from an annual mean of 11.0–22.3 mg m⁻². Dry-season standing stocks were 18% higher than during the wet season when spate-induced disturbance reduced algal standing stocks. Algal biomass varied significantly at the stream scale, but not at the pool scale, and was lower in unshaded streams, where standing stocks may have been limited by high densities of algivorous balitorid loaches (mainly Pseudogastromyzon myersi). An overriding effect of grazers on algal biomass could also have reduced variations resulting from spate-induced disturbance. Significant differences in assemblage composition among streams, which were dominated by diatoms and cyanobacteria (totally 82 taxa) were not systematically related to shading conditions. Seasonal variations in algal assemblages were statistically significant but rather minor, and did not involve major shifts in composition or growth form caused by spate-induced disturbance. The abundance of filamentous cyanobacteria in all the streams may have been due to ‘gardening’ by balitorid loaches that removed erect or stalked diatoms and favoured cyanobacteria that persist through basal regeneration of filaments. This explanation requires validation through manipulative experiments. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Handling editor: Luis Mauricio Bini