A persistent species assemblage structure along a Hawaiian stream from catchment-to-sea

In Limahuli Stream on the Hawaiian island of Kauai, species assemblage structure was monitored from catchment-to-sea over a 6-year period to determine how individual species utilized the stream continuum spatiotemporally. A persistent pattern was identified in which ten fish and macroinvertebrate species (nine native and one alien) were distributed into distinct zones of highest abundances with overlapping species ranges. Species diversity, as quantified by a dominance measure (Berger–Parker Index), was highest and least variable at the midpoint of the continuum where upstream–downstream species’ ranges converged. Reciprocal fluctuations in the population abundances of dominant species limited overall variation in species diversity to a 22% range which was interpreted as evidence of spatiotemporal persistence of the species assemblage structure. The pattern was captured in a testable, conceptual model which partitions an idealized Hawaiian stream from catchment-to-sea into five functional zones (Estuarine, Lower–Middle–Upper Reach, and Headwaters) positioned as percentages of continuum length. This model may be overlaid upon other Hawaiian streams for testing its applicability as well as to ask a variety of ecological questions about the manner in which species partition habitat spatiotemporally along biophysical gradients.

The effects of nutrient enrichment on a freshwater meiofaunal assemblage

1. The effects of eutrophication on phytoplankton, zooplankton and fish in lakes are well known. By contrast, little is known about the response of the zoobenthos to nutrient enrichment, while smaller organisms, such as the meiofauna, have for the most part been neglected. 2. In a long‐term (16 months) microcosm experiment, we assessed the effects of five levels of nutrients [total phosphorus (TP), 7–250 μg L^?1; nitrate, 2–8 mg L^?1] on a freshwater meiofaunal assemblage and on nematode diversity in particular. 3. Within the first 8 months, meiofaunal succession was only weakly affected, whereas, during the last 4 months, nutrient addition influenced most of the main taxa, with a concomitant change in the assemblage structure. 4. The density of the numerically dominant nematodes decreased upon nutrient enrichment, whereas ostracods became more numerous. Other taxa, including copepods, reached a maximum at intermediate nutrient levels or, in case of oligochaetes, were almost unaffected by nutrient enrichment. However, the changes in the density of the main taxa were usually insufficient to alter their biomass. Consequently, meiofaunal biomass was remarkably unresponsive to nutrient addition, while meiofaunal density displayed a unimodal relationship, with a peak at a TP concentration of 30 μg L^?1. In addition, nematode species richness decreased significantly with increasing nutrient concentrations. 5. We hypothesise that the response of meiofaunal taxa to nutrients is attributable to the development of primary producers, which shifted with enrichment from low densities of edible diatoms and unicellular green algae to large standing stocks of inedible forms, such as Lemna minor and Cladophora spp.     

Larval salamander growth responds to enrichment of a nutrient poor headwater stream

While many studies have measured effects of nutrient enrichment on higher trophic levels in grazing food webs, few such studies exist for detritus-based systems. We measured effects of nitrogen and phosphorus addition on growth of larval Eurycea wilderae in a heterotrophic headwater stream using a repeated mark-recapture design. Growth estimates for 208 recaptured larvae (control stream n = 92; treatment stream n = 116) resulted in a growth rate of 0.0027 d⁻¹ in each stream prior to enrichment, whereas during enrichment treatment growth rates (g = 0.0069 d⁻¹ [±0.0019, 95% C.I.]) were significantly higher than control (g = 0.0043 d⁻¹ [±0.0007, 95% C.I.]). Results indicate that E. wilderae growth is tightly linked to the detrital resource and that growth may be indirectly affected by both quantity and quality of detritus. This study provides some of the first evidence that nutrient enrichment of detritus-based systems can influence multiple trophic levels in ways similar to autotrophic systems.     

A comparison of algal,macroinvertebrate, and fish assemblage indices for assessing low-level nutrient enrichment in wadeable Ozark streams

Biotic indices for algae, macroinvertebrates, and fish assemblages can be effective for monitoring stream enrichment, but little is known regarding the value of the three assemblages for detecting perturbance as a consequence of low-level nutrient enrichment. In the summer of 2006, we collected nutrient and biotic samples from 30 wadeable Ozark streams that spanned a nutrient-concentration gradient from reference to moderately enriched conditions. Seventy-three algal metrics, 62 macroinvertebrate metrics, and 60 fish metrics were evaluated for each of the three biotic indices. After a group of candidate metrics had been identified with multivariate analysis, correlation procedures and scatter plots were used to identify the four metrics having strongest relations to a nutrient index calculated from log transformed and normalized total nitrogen and total phosphorus concentrations. The four metrics selected for each of the three biotic indices were: algae—the relative abundance of most tolerant diatoms, the combined relative abundance of three species of Cymbella, mesosaprobic algae percent taxa richness, and the relative abundance of diatoms that are obligate nitrogen heterotrophs; macroinvertebrate—the relative abundance of intolerant organisms, Baetidae relative abundance, moderately tolerant taxa richness, and insect biomass; fish—herbivore and detritivore taxa richness, pool species relative abundance, fish catch per unit effort, and black bass (Micropterus spp.) relative abundance.All three biotic indices were negatively correlated to nutrient concentrations but the algal index had a higher correlation (rho = ?0.89) than did the macroinvertebrate and fish indices (rho = ?0.63 and ?0.58, respectively). Biotic index scores were lowest and nutrient concentrations were highest for streams with basins having the highest poultry and cattle production. Because of the availability of litter for fertilizer and associated increases in grass and hay production, cattle feeding capacity increases with poultry production. Studies are needed that address the synergistic effect of poultry and cattle production on Ozark streams in high production areas before ecological risks can be adequately addressed.     

The effects of hydropsychid colonization on algal response to nutrient enrichment in a small Michigan stream, U.S.A.

1. We tested the hypothesis that the indirect effects of colonization by Hydropsyche spp. (Trichoptera: Hydropsychidae) may be greater than direct effects of nutrients on the benthic algal community growth. Two sets of nutrient-releasing substrates (a total of twenty-four) were deployed into a small pristine stream in northern Michigan. Each set was composed of four treatments replicated three times: (i) no nutrient enrichment (C), (ii) 0.5 M phosphate-P enrichment (P), (iii) 0.5 M nitrate-N enrichment (N) and (iv) 0.5 M phosphate-P plus 0.5 M nitrate-N enrichment (P + N). All hydropsychids colonizing on the substrate in one set (twelve substrates) were removed regularly and the other set (twelve substrates) with undisturbed hydropsychids served as the controls. 2. Algal biomass and gross primary productivity were estimated as chlorophyll a (chl a) concentration, algal biovolume, and carbon fixation rate, respectively. There was a significant interactive effect of hydropsychid colonization and P enrichment on algal biomass measured as chl a concentration. With removal of hydropsychids, chl a concentration increased 11-fold in the P enrichment treatments relative to the controls. The effects of P on chl a was, however, not significant in the presence of hydropsychids. Such interactive effects were not observed when algal responses were measured as biovolume and carbon fixation rate (GPP). 3. It is recommended that algal responses to nutrient enrichment should be measured as biovolume or carbon fixation rate in small streams where hydropsychids are commonly present.     

Deviation from strict homeostasis across multiple trophic levels in an invertebrate consumer assemblage exposed to high chronic phosphorus enrichment in a Neotropical stream

A central tenet of ecological stoichiometry is that consumer elemental composition is relatively independent of food resource nutrient content. Although the P content of some invertebrate consumer taxa can increase as a consequence of P-enriched food resources, little is known about how ecosystem nutrient loading can affect the elemental composition of entire consumer assemblages. Here we examine the potential for P enrichment across invertebrate consumer assemblages in response to chronic high P loading. We measured elemental ratios in invertebrate consumers and basal food resources in a series of streams in lowland Costa Rica that range widely in P levels (2-135 μg l^?1 soluble reactive P). Streams with high P levels receive natural long-term (over millennia) inputs of solute-rich groundwater while low-P streams do not receive these solute-rich groundwater inputs. P content of leaf litter and epilithon increased fourfold across the natural P gradient, exceeding basal resource P content values reported in the literature from other nutrient-rich streams. Invertebrate consumers from the high-P study stream were elevated twofold in P content across multiple taxonomic and functional feeding groups, including predators. Our results strongly support the hypothesis that elevated P content in consumers feeding on P-enriched food resources is a consequence of deviation from strict homeostasis. In contrast to prior studies, we found that between-stream variation in P content of a given taxon greatly exceeded within-stream variation among different taxa, suggesting that environment may be as important as phylogeny in controlling consumer stoichiometry. Relaxing the assumption of strict homeostasis presents challenges and opportunities for advancing our understanding of how nutrient limitation affects consumer growth. Moreover, our findings may provide a window into the future of how chronic anthropogenic nutrient loading can alter stoichiometric relationships in food webs.     

Efficacy of algal metrics for assessing nutrient and organic enrichment in flowing waters

1. Algal-community metrics were calculated for periphyton samples collected from 976 streams and rivers by the U.S. Geological Survey's National Water-Quality Assessment (NAWQA) Programme during 1993–2001 to evaluate national and regional relations with water chemistry and to compare whether algal-metric values differ significantly among undeveloped and developed land-use classifications.
2. Algal metrics with significant positive correlations with nutrient concentrations included indicators of trophic condition, organic enrichment, salinity, motility and taxa richness. The relative abundance of nitrogen-fixing algae was negatively correlated with nitrogen concentrations, and the abundance of diatom species associated with high dissolved oxygen concentrations was negatively correlated with both nitrogen and phosphorus concentrations. Median algal-metric values and nutrient concentrations were significantly lower at undeveloped sites than those draining agricultural or urban catchments.
3. Total algal biovolume did not differ significantly among major river catchments or land-use classifications, and was only weakly correlated with nitrate (positive) and suspended-sediment (negative) concentrations. Estimates of periphyton chlorophyll a indicated an oligotrophic–mesotrophic boundary of about 21 mg m⁻² and a mesotrophic–eutrophic boundary of about 55 mg m⁻² based on upper and lower quartiles of the biovolume data distribution.
4. Although algal species tolerance to nutrient and organic enrichment is well documented, additional taxonomic and autecological research on sensitive, endemic algal species would further enhance water-quality assessments.     

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.     

Recovery of three arctic stream reaches from experimental nutrient enrichment

1. Nutrient enrichment and resulting eutrophication is a widespread anthropogenic influence on freshwater ecosystems, but recovery from nutrient enrichment is poorly understood, especially in stream environments. We examined multi‐year patterns in community recovery from experimental low‐concentration nutrient enrichment (N + P or P only) in three reaches of two Arctic tundra streams (Kuparuk River and Oksrukuyik Creek) on the North Slope of Alaska (U.S.A.). 2. Rates of recovery varied among community components and depended on duration of enrichment (2–13 consecutive growing seasons). Biomass of epilithic algae returned to reference levels rapidly (within 2 years), regardless of nutrients added or enrichment duration. Aquatic bryophyte cover, which increased greatly in the Kuparuk River only after long‐term enrichment (8 years), took 8 years of recovery to approach reference levels, after storms had scoured most remnant moss in the recovering reach. 3. Multi‐year persistence of bryophytes in the Kuparuk River appeared to prevent recovery of insect populations that had either been positively (e.g. the mayfly Ephemerella, most chironomid midge taxa) or negatively (e.g. the tube‐building chironomid Orthocladius rivulorum) affected by this shift in dominant primary producer. These lags in recovery (of >3 years) were probably driven by the persistent effect of bryophytes on physical benthic habitat. 4. Summer growth rates of Arctic grayling (both adults and young‐of‐year) in Oksrukuyik Creek (fertilised for 6 years with no bryophyte colonisation), which were consistently increased by nutrient addition, returned to reference rates within 1–2 years. 5. Rates of recovery of these virtually pristine Arctic stream ecosystems from low‐level nutrient enrichment appeared to be controlled largely by duration of enrichment, mediated through physical habitat shifts caused by eventual bryophyte colonisation, and subsequent physical disturbance that removed bryophytes. Nutrient enrichment of oligotrophic Arctic stream ecosystems caused by climate change or local anthropogenic activity may have dramatic and persistent consequences if it results in the colonisation of long‐lived primary producers that alter physical habitat.     

人工遮光和营养添加对河流反硝化活性和反硝化细菌群落结构的影响

河流生态系统受到人类活动例如河岸带森林植被毁损和农业活动施肥等的干扰日益加剧,而这些活动使河流接收的光照增多、河流的氮磷营养盐浓度增加。微生物的反硝化作用是河流去除氮的有效途径。在汉江的一级支流金水河上游核心保护区内选取6条溪流开展野外控制实验,利用营养添加模拟河流中营养的增加,遮盖河面来模拟源头溪流的隐蔽状态,来研究河流沉积物中微生物的反硝化作用对光照和营养改变的响应,并利用高通量测序(Mi Seq)技术研究在两种处理下河流沉积物中nir S型反硝化细菌的群落结构变化。结果显示:营养元素添加促进了沉积物的反硝化活性,河面遮盖抑制了沉积物的反硝化活性。营养添加和遮盖两种处理均降低了控制实验区域内脱氯单胞菌属(优势菌属)的相对丰度,同时也降低了该区域nir S型反硝化菌群落的Chao多样性。本研究初步证实了光照增加和河流的营养增加提高了河流沉积物反硝化活性,并为提高河流的脱氮能力提供科学依据。     

Response of zooplankton to nutrient enrichment and fish in shallow lakes: a pan-European mesocosm experiment

1. Responses of zooplankton to nutrient enrichment and fish predation were studied in 1998 and 1999 by carrying out parallel mesocosm experiments in six lakes across Europe. 2. Zooplankton community structure, biomass and responses to nutrient and fish manipulation showed geographical and year‐to‐year differences. Fish had a greater influence than nutrients in regulating zooplankton biomass and especially the relative abundances of different functional groups of zooplankton. When fish reduced the biomass of large crustaceans, there was a complementary increase in the biomasses of smaller crustacean species and rotifers. 3. High abundance of submerged macrophytes provided refuge for zooplankton against fish predation but this refuge effect differed notably in magnitude among sites. 4. Large crustacean grazers (Daphnia, Diaphanosoma, Sida and Simocephalus) were crucial in controlling algal biomass, while smaller crustacean grazers and rotifers were of minor importance. Large grazers were able to control phytoplankton biomass even under hypereutrophic conditions (up to 1600 μg TP L^?1) when grazer biomass was high (>80–90 μg dry mass L^?1) or accounted for >30% of the grazer community. 5. The littoral zooplankton community was less resistant to change following nutrient enrichment in southern Spain, at high temperatures (close to 30 °C), than at lower temperatures (17–23 °C) characterising the other sites. This lower resistance was because of a greater importance of nutrients than zooplankton in controlling algal biomass. 6. Apart from the reduced role of large crustacean grazers at the lowest latitude, no consistent geographical patterns were observed in the responses of zooplankton communities to nutrient and fish manipulation.     

Effects of nutrient enrichment and leaf quality on the breakdown of leaves in a hardwater stream

1. The breakdown of leaf litter in streams is influenced strongly by leaf quality and the concentration of dissolved nutrients, primarily inorganic nitrogen (N) and phosphorus (P) in the water. We examined the effect of nutrient enrichment on the breakdown of three species of leaves in a hardwater, nutrient‐rich stream. The rate of microbial respiration was also measured on the decomposing leaves. 2. The breakdown rates of dogwood (Cornus stolonifera), aspen (Populus tremuloides) and birch (Betula occidentalis), k‐values of 0.0461, 0.0307 and 0.0186 day^–1, respectively, were unaffected by nutrient enrichment and generally faster than reported previously. Microbial respiration on the leaves was greater than reported previously for leaves of congeneric species. It appears that leaf breakdown in the study stream was not nutrient limited. 3. Nitrogen‐based measures of leaf quality, such as percentage N and carbon (C)/nitrogen ratio, did not correspond to measured breakdown rates among the three leaf types. The best predictors of relative breakdown rates were percentage lignin and the percentage of the total carbon that occurred as lignin. We suggest that, when leaf breakdown is not nutrient limited, measures of carbon quality (i.e. lignin‐based measures) are a better assessment of overall leaf quality than are N‐based measures. 4. Previous studies have indicated that the enzymes produced by aquatic hyphomycetes (microfungi) operate most efficiently at a basic pH and in the presence of calcium ions. The hardwater conditions (pH=8.6, total hardness > 300 mg CaCO₃ L^–1) and abundance of dissolved NO₃ and soluble reactive phosphorous (SRP) (approximately 50 μg L^–1, each) in the study stream appear to have provided conditions that resulted in a high respiration rate and rapid breakdown of leaf litter.     

河流着生藻类多样性对城镇化的响应——以深圳市为例

着生藻类是河流生态状况的重要指示生物,然而关于城镇化对着生藻类多样性影响的研究还较缺乏。基于深圳市9条主要河流74个样点2020年雨、旱季的调查数据,对比了城区、郊区河流样点的着生藻类α、β多样性。两次调查共鉴定出着生藻类301个分类单元,其中硅藻门种类最多。谷皮菱形藻(Nitzschia palea)、隐头舟形藻(Navicula cryptocephala)、平庸菱形藻(Nitzschia inconspicua)等污染指示种在城区组相对丰度更高,而清洁指示种微小曲丝藻(Achnanthidium minutissimum)在郊区组相对丰度更高。城镇化对着生藻类物种丰富度、香农维纳多样性指数、Pielou均匀度指数等α多样性指数均无显著影响,但对着生藻类群落结构有显著影响。城区组样点的着生藻类β多样性(用主坐标分析质心指示)低于郊区组,城区组与郊区组藻类β多样性差异在旱季更为显著。两组样点的藻类β多样性差异主要来源于物种周转组分。与城区组着生藻类群落显著相关的环境因子包括总氮、高锰酸钾指数、电导率、pH、浊度和水深,而与郊区组着生藻类群落显著相关的是总氮、溶解性无机磷和电导率。研究...     

Microhabitat use in a stream fish assemblage

We examined microhabitat use among fishes in a 37-m section of Coweeta Creek, North Carolina. Numerical abundances of species changed substantially during the 17-month study period. Microhabitat availability, however, did not change markedly during our investigation. Seasonal principal component analyses of microhabitat availability and fish microhabitat use data extracted two main patterns of non-random microhabitat use. Clinostomus funduloides, Notropis eoccogenis and Semotilus atromaculatm were significantly over-represented in deep areas with low to intermediate velocities and depositional substrata. Campostoma oligolepis, Coitus bairdi, Etheostoma blennioides, Rhinichthys cataractae and Salmo gairdneri all occurred in intermediate to deep microhabitats with moderate to high velocities and erosional substrata. Five of seven species exhibited seasonal variation in microhabitat utilization, whereas six species displayed size-related variation in use. Size-related variation was probably ontogenetic. We attributed most seasonal changes in microhabitat use to variations in microhabitat availability.
We used canonical analysis of discriminants to identify factors maximizing interspecific differences in microhabitat use. This analysis indicated that species could be assigned to either a benthic or a water column guild. Species within a guild generally could not be differentiated statistically, whereas members of different guilds were readily separable. These patterns persisted throughout the study, despite changes in numerical abundances of assemblage members. There was no evidence of either exploitation or interference competition for microhabitat, consequently it is unlikely that spatial resources were limiting during our study.     

Separating physical disturbance and nutrient enrichment caused by Pacific salmon in stream ecosystems

1. Pacific salmon (Oncorhynchus spp.) deliver marine‐derived nutrients to the streams in which they spawn and die, and these resource subsidies can increase the abundance of stream biota. In strong contrast, physical disturbance from salmon spawning activity can reduce the abundance of benthic organisms. Previous experimental designs have not established the relative effects of these two contrasting processes on stream organisms during a salmon run. 2. We combined manipulative and observational field studies to assess the degree of nutrient enrichment, physical disturbance, and the net effect of salmon on the abundance of benthic periphyton. Related salmon‐mediated processes were also evaluated for benthic macroinvertebrates. Mesh exclosures (2 × 2 m plots) prevented salmon from disturbing areas of the stream channel, which were compared with areas to which salmon had access. Sampling was conducted both before and during the late‐summer spawning run of pink (O. gorbushca) and chum (O. keta) salmon. 3. Streamwater nitrogen and phosphorus concentrations increased sharply with the onset of the salmon run, and highly significant positive relationships were observed between the numbers of salmon present in the stream and these dissolved nutrients. Before the salmon run, periphyton biomass (as chlorophyll a) and total macroinvertebrate abundance were very similar between control and exclosure plots. During the salmon run, exclosures departed substantially from controls, suggesting significant disturbance imparted on benthic biota. 4. Comparing exclosures before and during the salmon run enabled us to estimate the effects of salmon in the absence of direct salmon disturbance. This ‘nutrient enrichment potential’ was significant for periphyton biomass, as was a related index for macroinvertebrate abundance (although enhanced invertebrate drift into exclosures during the salmon run could also have been important). Interestingly, however, the net effect of salmon, evaluated by comparing control plots before and during the salmon run, was relatively modest for both periphyton and macroinvertebrates, suggesting that nutrient enrichment effects were largely offset by disturbance. 5. Our results illustrate the importance of isolating the specific mechanisms via which organisms affect ecosystems, and indicate that the relative magnitude of salmon nutrient enrichment and benthic disturbance determines the net effect that these ecologically important fish have on stream ecosystems.     

Effect of macrophyte community composition and nutrient enrichment on plant biomass and algal blooms

Submerged freshwater macrophytes decline with increasing eutrophication. This has consequences for ecosystem processes in shallow lakes and ponds as macrophytes can reduce algal blooms under eutrophic conditions. We hypothesize that the productivity of submerged vegetation, biomass change under eutrophication and the suppression of algal blooms may be affected by macrophyte community composition. To test our hypothesis, we established three macrophyte community types in 36 fishless experimental ponds: one dominated by the oligotrophic species Chara globularis, one dominated by the eutrophic species Potamogeton pectinatus and a diverse vegetation which became co-dominated by Elodea nuttallii and C. globularis, and we fertilized half of the ponds.The macrophyte communities produced different amounts of biomass and they responded differently to fertilization. The community dominated by Potamogeton produced the lowest overall biomass, but was not affected by nutrient addition. The communities dominated by Chara and co-dominated by Elodea and Chara produced more than four-fold the amount of biomass produced in Potamogeton communities under oligotrophic conditions, but were strongly negatively affected by nutrient addition.Phytoplankton abundance did not differ significantly among the plant community types, but showed large variation within community types. There was a significant negative relationship between spring macrophyte biomass and the probability of summer algal blooms. The occurrence of algal blooms coincided with low daphnid densities and high pH (>10).We conclude that the macrophyte community composition, characterized by the dominant species, strongly affected the amount of biomass production as well as the short-term response of the vegetation to nutrient enrichment. Macrophyte community composition had no direct effect on algal blooms, but can affect the occurrence of algal blooms indirectly as these occurred only in ponds with low (<100 g/m² DW) spring macrophyte biomass.     

The response of planktonic and microbenthic algal assemblages to nutrient enrichment in shallow coastal waters,southwest Sweden

Field and laboratory nutrient (nitrogen and phosphorus) enrichment experiments were performed using natural phytoplankton and microphytobenthic assemblages from the brackish water Öresund, S.W. Sweden. The response of algae from a low-nutrient area (Falsterbo Canal) was compared to that of algae from a polluted, nutrient-rich area (Lomma Bay).The biomass (measured as chlorophyll a) of both phytoplankton and microphytobenthos from the Falsterbo Canal increased after the addition of nitrogen. Phytoplankton growth was stimulated by the addition of phosphorus to the nitrogen-rich water of the polluted Lomma Bay. Sediment chlorophyll a showed no significant increase after the addition of nutrients in the Lomma Bay. In containers without sediment, phytoplankton uptake was calculated to account for ≈ 90% of the disappearance of inorganic fixed nitrogen from the water. In the sediment containers the microphytobenthos was estimated to account for ≈20% of the nitrogen uptake. The rest was presumably lost mainly through denitrification.When containers with microphytobenthos from Lomma Bay were kept in the dark, phosphorus was released at a rate of up to ≈ 180 μM · m^?2 · day^?1. We suggest that by producing oxygen microbenthic algae keep the sediment surface oxygenated thereby decreasing phosphorus transport from the sediment to the overlying water.     

Reference diatom assemblage response to restoration of an acid mine drainage stream

A prevalent legacy of coal mining within Appalachia and elsewhere is acid mine drainage (AMD), which drastically alters both the chemical and biological components of the receiving waters. Hewett Fork is one such affected stream. Although AMD treatment has reduced acidity considerably downstream, the ability of this stream to sustain a biological community compared to those found in reference conditions remains unclear. To assess this, tiles colonized with diatom assemblages from a reference stream were transplanted into Hewett Fork in 5 locations along a 6.9 km stream length and sampled after one, three, and six weeks. Diatom assemblage structure metrics, including species evenness (J’), species richness (S), relative abundance of dominant taxon, and Shannon diversity (H′), as well as chlorophyll a concentrations, Bray–Curtis dissimilarities, and Acid Mine Drainage Diatom Index of Biotic Integrity (AMD-DIBI) scores were calculated for each site and sampling time. One-way ANOVAs of structural metrics showed significant differences (P ≤ 0.001) between the reference site and the 2nd and 3rd most upstream sites within the study reach for the duration of the study, with the exception of the relative abundance of dominant taxa at an intermediate site during the third week. Conversely, the most downstream Hewett Fork assemblage, located 11.6 km from the primary AMD input, did not differ significantly (P > 0.05) from that of the reference assemblage for any structural metrics after the initial sampling period. Throughout the study, only three sites obtained “good” AMD-DIBI narrative class:the reference site (weeks 1, 3, and 6), the most downstream site (weeks 1, 3, and 6; 11.6 km downstream of primary AMD input) and the uppermost site (weeks 1 and 6; 4.7 km downstream of primary AMD input). Results suggested that after an initial one-week acclimation period, assemblages at the uppermost and most downstream sites along the reach were relatively similar to those found in reference conditions, while sites within the middle region continued to show signs of impairment, although the factor(s) causing this impairment remain unknown. These findings suggest that while treatment has been effective on a site-specific basis, the expected linear-response to treatment was not achieved due to underlying factors that are inhibiting reference-like biological communities from reestablishing within the affected stream reach.     

Periphyton nutrient content,biomass and algal community on artificial substrate: response to experimental nutrient enrichment and the effect of its interruption in a tropical reservoir

Periphyton plays an important functional role in the retention of nutrients in aquatic ecosystems, especially phosphorus. We evaluated the effects of enrichment with N and P and the effect after 20 days of no additional N and P on periphyton on artificial substratum in open-bottom mesocosms. The aim was to jointly evaluate periphyton, phytoplankton and zooplankton in the presence of macrophytes. Experimental conditions simulated natural conditions and nutrient addition was based on the maximum concentration recorded in mesotrophic reservoir. Our hypothesis is that the periphyton is sensitive to the effects of N and P enrichment and its interruption, despite the positive response of phytoplankton and zooplankton. Two treatments were designed using open-bottom mesocosms (n = 3): control (no nutrient addition); NP+ (combined phosphorus and nitrogen addition). Sampling for the measurement of biotic and abiotic variables was performed, with 10 days of continuous enrichment, on the 3rd, 6th and 11th, and 20 days after enrichment had ended (31st day). Periphyton chlorophyll a, dry mass and algal density increased significantly with the addition of N and P and decreased 20 days after the interruption of the enrichment. The highest periphyton P content was found in the NP+ treatment. The enrichment had a positive effect on Chrysophyceae (Chromulina spp.) and rotifer (Polyarthra spp.) density and the interruption of enrichment favored Bacillariophyceae (Gomphonema sp.) and rotifers (Gastropus stylifer). Phytoplankton responded positively to enrichment. Along with the high macrophyte coverage over the experimental period, we evidenced the positive effect enrichment had on phytoplankton biomass and zooplankton abundance. Therefore, periphyton on artificial substrate was sensitive to effects of N and P enrichment and its interruption, responding promptly to changes in nutrient availability in a scenario of high competition and grazing.     

Resource partitioning in a tropical stream fish assemblage

An assemblage of freshwater fishes inhabiting a medium-sized stream in tropical central Vietnam was investigated with respect to spatial and trophic distribution. Point-abundance sampling, gut content analysis and relative intestine length yielded interspecific differences in niche expression. Conspecific juvenile and adult habitat and feeding niches were also distinct. Niche differences arose mainly from differences in current velocity, substrata and foraging preferences. Extreme specializations, such as selective feeding on hard incustrations of cyanobacteria by adult Annamia normani , were present. The low niche overlap suggests that this tropical fish assemblage is structured by competitive interactions.