Biotic and abiotic controls on the ecosystem significance of consumer excretion in two contrasting tropical streams

1. Excretion of nitrogen (N) and phosphorus (P) is a direct and potentially important role for aquatic consumers in nutrient cycling that has recently garnered increased attention. The ecosystem‐level significance of excreted nutrients depends on a suite of abiotic and biotic factors, however, and few studies have coupled measurements of excretion with consideration of its likely importance for whole‐system nutrient fluxes. 2. We measured rates and ratios of N and P excretion by shrimps (Xiphocaris elongata and Atya spp.) in two tropical streams that differed strongly in shrimp biomass because a waterfall excluded predatory fish from one site. We also made measurements of shrimp and basal resource carbon (C), N and P content and estimated shrimp densities and ecosystem‐level N and P excretion and uptake. Finally, we used a 3‐year record of discharge and NH₄‐N concentration in the high‐biomass stream to estimate temporal variation in the distance required for excretion to turn over the ambient NH₄‐N pool. 3. Per cent C, N, and P body content of Xiphocaris was significantly higher than that of Atya. Only per cent P body content showed significant negative relationships with body mass. C:N of Atya increased significantly with body mass and was higher than that of Xiphocaris. N : P of Xiphocaris was significantly higher than that of Atya. 4. Excretion rates ranged from 0.16–3.80 μmol NH₄‐N shrimp^?1 h^?1, 0.23–5.76 μmol total dissolved nitrogen (TDN) shrimp^?1 h^?1 and 0.002–0.186 μmol total dissolved phosphorus (TDP) shrimp^?1 h^?1. Body size was generally a strong predictor of excretion rates in both taxa, differing between Xiphocaris and Atya for TDP but not NH₄‐N and TDN. Excretion rates showed statistically significant but weak relationships with body content stoichiometry. 5. Large between‐stream differences in shrimp biomass drove differences in total excretion by the two shrimp communities (22.3 versus 0.20 μmol NH₄‐N m^?2 h^?1, 37.5 versus 0.26 μmol TDN m^?2 h^?1 and 1.1 versus 0.015 μmol TDP m^?2 h^?1), equivalent to 21% and 0.5% of NH₄‐N uptake and 5% and <0.1% of P uptake measured in the high‐ and low‐biomass stream, respectively. Distances required for excretion to turn over the ambient NH₄‐N pool varied more than a hundredfold over the 3‐year record in the high‐shrimp stream, driven by variability in discharge and NH₄‐N concentration. 6. Our results underscore the importance of both biotic and abiotic factors in controlling consumer excretion and its significance for nutrient cycling in aquatic ecosystems. Differences in community‐level excretion rates were related to spatial patterns in shrimp biomass dictated by geomorphology and the presence of predators. Abiotic factors also had important effects through temporal patterns in discharge and nutrient concentrations. Future excretion studies that focus on nutrient cycling should consider both biotic and abiotic factors in assessing the significance of consumer excretion in aquatic ecosystems.     

Empirical models for predicting the excretion of nutrients (N and P) by aquatic metazoans: taxonomic differences in rates and element ratios

1. We developed empirical models for predicting the release of nutrients [nitrogen (N) and phosphorus (P)] by aquatic metazoans (zooplankton, mussels, benthic macroinvertebrates and fish). 2. The number of species represented in each model ranged from 9 to 74 (n = 40 – 1122), organism dry mass from 1 × 10^?5 to 8 × 10⁴ mg and water temperature from ?1.8 to 32 °C for all models. Organisms were from marine and freshwater (both lotic and lentic) environments. 3. Rates and ratios of nutrient excretion were modelled and intra‐ and intertaxon differences in excretion were examined. Rates of N and P excretion were not significantly different between marine and freshwater species within the same taxon (e.g. zooplankton). However, rates of excretion (as a function of organism dry mass and water temperature) were significantly different among different orders of zooplankton, mussels and fish. However, excretion of N was similar among different orders of benthic macroinvertebrates. 4. Detritivorous fish excreted both N and P at rates greater than all other taxa; whereas mussels excreted N and P generally at rates less than other taxa. There were no significant differences in the rate of N and P excretion between zooplankton and fish (i.e. the allometry of N and P excretion was similar between zooplankton and fish). 5. Molar N : P ratios of nutrients excreted increased with increasing organism dry mass for each group of metazoans, except for zooplankton and detritivorous fish (where N : P ratios declined with increasing organism dry mass). Molar N : P ratios in the excretions of aquatic metazoans were generally below the Redfield ratio of 16:1. 6. We examined the influence of variable abundance of zooplankton, benthic macroinvertebrates and fish on assemblage excretion rates. Rates of N and P excretion were calculated by applying our models to metazoan biomass and abundance data over seven consecutive years in two oligotrophic lakes. Rates of N and P excretion (g ha^?1 day^?1) increased linearly with increasing assemblage biomass (kg ha^?1). However, rates of N and P excretion were significantly and negatively correlated with the relative abundance of fish and positively correlated with the relative abundance of zooplankton.     

Impact of the removal of gizzard shad (Dorosoma cepedianum) on nutrient cycles in Lake Apopka,Florida

1. The St. Johns River Water Management District removed over 5.4 million kg of gizzard shad (Dorosoma cepedianum) from Lake Apopka, FL during 1993–2005, as a means of reducing lake phosphorus and phytoplankton concentrations and improving water clarity. Other steps included reduction of external nutrient inputs and operation of a treatment wetland. We measured nutrient excretion by Lake Apopka gizzard shad to quantify the nutrient effect of this biomanipulation. 2. Both N and P excretion were significantly affected by fish body mass and temperature. Larger fish had lower mass‐specific rates of excretion than smaller fish. 3. High water temperature increased P excretion to a much greater extent than N, resulting in a low N : P of nutrient excretion in midsummer. The N : P of excretion was lower than has been observed in other systems, probably because of higher water temperature. 4. Removal of gizzard shad >200 g prevented the annual release of 45 800 kg N year^?1 (3.46 kg N ha^?1 year^?1) and 7700 kg P year^?1 (0.62 kg P ha^?1 year^?1) on average. The actual impact on the P cycle varied substantially from year to year (range 7900–78 800 kg N year^?1; 1200–14 800 kg P year^?1), primarily because of fluctuations in the catch. 5. On an annual basis, the P directly removed in fish tissues was similar to that removed by the treatment wetland. The P excretion prevented by the removal of fish was approximately 20% of the reduction in external P loading achieved during 1993–2005. 6. In the short term, most of the P demand of planktonic primary producers is met through recycling of P, which greatly exceeds external P loading. Depending on population biomass, phosphorus excretion by the resident gizzard shad population was similar in magnitude to the P release by diffusive flux from the sediments.     

Release rates and potential fates of nitrogen and phosphorus from sediments in a eutrophic reservoir

1. Nutrients released from lake sediments can influence water column nutrient concentrations and planktonic productivity. We examined sediment nutrient release [soluble reactive phosphorus (SRP) and ammonia (NH)] at two sites in a eutrophic reservoir (Acton Lake, OH, U.S.A.) that differed in physical mixing conditions (a thermally stratified and an unstratified site). 2. Sediment nutrient release rates were estimated with three methods: sediment core incubations, seasonal in situ hypolimnetic accumulation and a published regression model that predicted sediment phosphorous (P) release rate from sediment P concentration. All three methods were applied to the deeper stratified site in the reservoir; however, we used only sediment core incubations to estimate SRP and NH release rates at the shallow unstratified site because of the lack of thermal stratification. We also compared the total P concentration (TP_S) of sediments and the concentration of P in various sediment fractions at both sites. 3. Anoxic sediments at the stratified site released SRP at rates more than an order of magnitude greater than oxic sediments at the shallow unstratified site. However, P accumulated in the hypolimnion at much lower rates than predicted by sediment core incubations. In contrast, NH was released at similar rates at both sites and accumulated in the hypolimnion at close to the expected rate, indicating that P was ‘lost’ from the hypolimnion through biogeochemical pathways for P, such as precipitation with inorganic material or biological uptake and sedimentation. 4. TP_S was significantly greater at the deeper stratified site and organically bound P accounted for >50% of TP_S at both sites. 5. We examined the magnitude of SRP fluxes into the study reservoir in 1996 by comparing the mean summer daily SRP fluxes from anaerobic sediments, aerobic sediments, stream inflows and gizzard shad excretion. While the SRP release from anaerobic sediments was high, we hypothesise that little of this SRP gained access to the epilimnion in mid‐summer. SRP flux to the reservoir from aerobic sediments was less than from gizzard shad excretion and streams. Large interannual variability in thermocline stability, gizzard shad biomass and stream discharge volumes, will affect SRP loading rates from different sources in different years. Therefore, construction of P budgets for different years should account for interannual variation in these parameters.     

Comparing Effects of Nutrients on Algal Biomass in Streams in Two Regions with Different Disturbance Regimes and with Applications for Developing Nutrient Criteria

Responses of stream algal biomass to nutrient enrichment were studied in two regions where differences in hydrologic variability cause great differences in herbivory. Around northwestern Kentucky (KY) hydrologic variability constrains invertebrate biomass and their effects on algae, but hydrologic stability in Michigan (MI) streams permits accrual of high herbivore densities and herbivory of benthic algae. Multiple indicators of algal biomass and nutrient availability were measured in 104 streams with repeated sampling at each site over a 2−month period. Many measures of algal biomass and nutrient availability were positively correlated in both regions, however the amount of variation explained varied with measures of biomass and nutrient concentration and with region. Indicators of diatom biomass were higher in KY than MI, but were not related to nutrient concentrations in either region. Chl a and % area of substratum covered by Cladophora were positively correlated to nutrient concentrations in both regions. Cladophora responded significantly more to nutrients in MI than KY. Total phosphorus (TP) and total nitrogen (TN) explained similar amounts of variation in algal biomass, and not significantly more variation in biomass than dissolved nutrient concentrations. Low N:P ratios in the benthic algae indicated N as well as P may be limiting their accrual. Most observed responses in benthic algal biomass occurred in nutrient concentrations between 10 and 30 μg TP l⁻¹ and between 400 and 1000 μg TN l⁻¹.     

PHYSIOLOGICAL INDICATORS OF NUTRIENT DEFICIENCY IN CLADOPHORA (CHLOROPHYTA) IN THE CLARK FORK OF THE COLUMBIA RIVER,MONTANA1

Cellular nutrient concentrations and nutrient uptake rates of Cladophora glomerata (L.) Kuetzing were determined during summer and fall in 1989–1990 at a site on the upper Clark Fork of the Columbia River, Montana. Both physiological tests indicated that Cladophora growth is likely to be limited by nitrogen during late summer-early fall. Maximum uptake rates of ammonia-N and nitrate-N were 5935–6991 and 507–984 μg · g DW^?1· h^?1, respectively, during July–October when dissolved inorganic nitrogen (DIN) concentrations in the river were less than 10 μg · L^?1. During November-December, when DIN was 72–376 μg · L^?1, maximum ammonia-N uptake was 1137–1633 μg · g DW^?1· h^?1 and maximum nitrate-N uptake was 0–196 μg · g DW^?1· h^?1. Cellular nitrogen during summer–early fall was 0.78–1.80% of Cladophora dry weight, frequently at or below 1.1%, a level suggested as a critical minimum N concentration for maximum growth. In contrast, cellular P was 0.18–0.36% of dry weight, 3–6 times the suggested critical P concentration of 0.06%. Molar ratios of cellular N:P (< 16:1) and DIN: SRP (< 4:1) during late summer-early fall also indicated potential N limitation. Cellular N and P from Cladophora collected from a second site influenced by a municipal wastewater discharge in 1990 displayed similar seasonal trends. At both sites, seasonal fluctuations in DIN were closely tracked by changes in cellular N, Cellular P, however, increased through the growing season despite declining levels of SRP in the river.     

Ontogenetic diet shifts produce trade‐offs in elemental imbalance in bluegill sunfish

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Consumer‐resource stoichiometry as a predictor of trophic discrimination (Δ13C, Δ15N) in aquatic invertebrates

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Invertebrate grazing and riparian shade as controllers of nuisance algae in a eutrophic river

1. Algal growth in lotic systems is controlled either from the bottom‐up (e.g. nutrients and light, which determine growth rates) or from the top‐down (e.g. grazing pressure, which reduces accumulated biomass). Nutrient‐enriched streams that support large and diverse grazing macroinvertebrate populations and those with shaded riparian corridors rarely suffer from excessive algal growth. 2. In this study, the density of benthic algivorous macroinvertebrates was experimentally manipulated in shaded and open nutrient‐enriched stream habitats of the Owennagearagh River, south‐west Ireland. The ability of macroinvertebrate grazers and riparian shade to control benthic algal growth [particularly the nuisance alga, Cladophora glomerata (L. Kütz)] was investigated. Three sites with markedly different concentrations of plant nutrients (one site upstream and two sites downstream of the sewage outfall) were selected. The density of grazing invertebrates colonising ceramic tiles was reduced using high‐voltage localised electric pulses. Replicates of treatment (grazer‐excluded) and control (grazed) tiles were deployed in open and shaded (<25 and >80% canopy cover, respectively) patches of stream bed, in each site. 3. After 2‐week Cladophora cover, periphytic chlorophyll a and biofilm ash‐free dry mass (AFDM) were quantified for all experimental tiles. Values for all three parameters were highest on grazer‐excluded tiles from open patches. Grazed tiles from open patches accrued little Cladophora and had significantly lower levels of chlorophyll a and AFDM. Nutrient inputs were found to have an impact on the density of grazing invertebrates, with higher densities of Baetis nymphs at the most nutrient‐enriched site. 4. Our results demonstrate that in eutrophic, high‐light streams, filamentous algae can quickly accumulate to nuisance levels in the absence of invertebrate grazers. In future, greater attention should be paid to the role of grazing invertebrates in controlling nuisance algae in streams, in addition to algal–nutrient relationships.     

Ecological responses to simulated benthic-derived nutrient subsidies mediated by omnivorous fish

1. Fish can play an important role in coupling benthic and pelagic habitats by consuming benthic prey and providing essential nutrients to algae in dissolved form. However, little is known about the factors affecting the magnitude of this nutrient subsidy. 2. Using laboratory and mesocosm experiments we evaluated how varying ingestion rates of bluegill sunfish (Lepomis macrochirus) affects fish excretion rates of both nitrogen (N) and phosphorus (P). During the 10‐week mesocosm experiment, we also evaluated how varying ingestion rates may affect plankton community dynamics, and nutrient flux between pelagic and benthic habitats. Lastly, bioenergetic/mass balance models were used to examine the nutrient stoichiometry of fish body composition and excretion products. 3. Under laboratory conditions, both N and P excretion rates increased with increased ingestion of benthic prey surrogates (earthworms). This effect was more pronounced for N than P. Furthermore, under the more realistic conditions of the mesocosm experiment ingestion rate had no significant effect on P excretion rate. 4. Increased fish ingestion rate in the mesocosm experiment increased total algal biomass and the flux of nutrients from the water column to sediments. Effects of variable ingestion were much stronger on periphyton biomass and algal sedimentation rates than on phytoplankton or zooplankton biomass or composition. 5. Fish body nutrient composition was greatly affected by ingestion rate. N content increased and P content decreased with ingestion rate. As a result, the N : P ratio of fish bodies also increased with ingestion rate. The N : P ratio of nutrients excreted by fish also increased with ingestion rate, counter to predictions of stoichiometric theory, which predicts that excreted N : P ratio is negatively correlated to body N : P. However, this finding can be explained by relaxing the assumption of constant nutrient assimilation rates, and our mass balance data suggest that assimilation rates vary indeed with ingestion rate. 6. Our study provides experimental evidence that translocation of benthic‐derived nutrients by fish can affect the flux of nutrients among habitats, while also suggesting that stoichiometry models need to better incorporate how variable ingestion rates affect nutrient assimilation and excretion rates.     

The trophic interactions of young Arctic grayling (Thymallus arcticus) in an Arctic tundra stream

1. Young (0+) Arctic grayling (Thymallus arcticus) have the potential to control the trophic structure of Arctic tundra streams through consumption, nutrient excretion and the modification of prey behaviour. The effect of young grayling on three trophic levels (algae, invertebrates and fish) was investigated by manipulating fish density and by fertilizing the river with phosphorus (P). 2. Nutrients, epilithic chlorophyll a, benthic invertebrates and fish biomass were measured within each fish density treatment (0, 4, and 40 fish m^–2) within the P-limited reference zone and the P-enriched fertilized zone of the Kuparuk River, Alaska. 3. Epilithic chlorophyll a increased with increased fish density in both reference and fertilized zones, while mayfly density decreased with increased fish density in the fertilized zone only. Final mean mass of young grayling in the 40 fish m^–2 cages was lower than mean mass in the 4 fish m^–2 cages. 4. Young grayling may produce a top-down cascading trophic effect in areas where nutrients are not limited. 5. River nutrient status and river discharge may modify the strength of top-down control by young grayling.     

Stoichiometry of benthic invertebrate nutrient recycling: interspecific variation and the role of body mass

Ecological stoichiometry (ES) and allometry offer frameworks for predicting how nutrient recycling varies within and among animal species. Despite the importance of benthic-derived nutrients in most aquatic systems, predictions based on ES and allometry have been poorly tested among benthic invertebrate consumers. Here, we show that the rates and ratios at which three freshwater benthic invertebrate species (a crustacean, an insect, and a polychaeta) recycled nitrogen (N) and phosphorus (P) can be partially predicted by ES and allometry depending on whether data are analyzed intra- or interspecifically. Mass-specific N and P excretion rates were negatively correlated with invertebrate body size both among and within taxa, supporting allometric predictions. However, mass-specific N and P excretion rates were positively and negatively correlated to invertebrate body N and P, respectively, but only when data were analyzed intraspecifically. As a corollary, the mass-specific N:P excretion ratio was positively related to body N:P ratio. Such a contrasting pattern on excretion-mediated N and P recycling suggests that stoichiometric constraints regarding consumer-resource imbalances for the three species utilized in this study may be stronger for P than for N. Our results indicate that the variation in nutrient recycling, which is mediated by taxonomic constraints on stoichiometry and allometry, may substantially help us to understand the importance of benthic detritivorous species to the functioning of aquatic ecosystems.     

An invasive frog, Eleutherodactylus coqui, increases new leaf production and leaf litter decomposition rates through nutrient cycling in Hawaii

A frog endemic to Puerto Rico, Eleutherodactylus coqui, invaded Hawaii in the late 1980s, where it can reach densities of 50,000 individuals ha⁻¹. Effects of this introduced insectivore on invertebrate communities and ecosystem processes, such as nutrient cycling, are largely unknown. In two study sites on the Island of Hawaii, we studied the top-down effects of E. coqui on aerial, herbivorous, and leaf litter invertebrates; herbivory, plant growth, and leaf litter decomposition rates; and leaf litter and throughfall chemistry over 6 months. We found that E. coqui reduced all invertebrate communities at one of the two study sites. Across sites, E. coqui lowered herbivory rates, increased NH₄⁺ and P concentrations in throughfall, increased Mg, N, P, and K in decomposing leaf litter, increased new leaf production of Psidium cattleianum, and increased leaf litter decomposition rates of Metrosideros polymorpha. In summary, E. coqui effects on invertebrates differed by site, but E. coqui effects on ecosystem processes were similar across sites. Path analyses suggest that E. coqui increased the number of new P. cattleianum leaves and leaf litter decomposition rates of M. polymorpha by making nutrients more available to plants and microbes rather than through changes in the invertebrate community. Results suggest that E. coqui in Hawaii has the potential to reduce endemic invertebrates and increase nutrient cycling rates, which may confer a competitive advantage to invasive plants in an ecosystem where native species have evolved in nutrient-poor conditions.     

Evaluating a sustainability index for nutrients in a short rotation energy cropping system

In dryland environments 3–5 year rotations of tree crops and agriculture represent a major potential bioenergy feedstock and a means to restore landscape hydrologic balances and phytoremediate sites, while maintaining food production. In soils with low natural fertility, the long‐term viability of these systems will be critically affected by site nutrient status and subsequent cycling of nutrients. A nutrient assimilation index (NAI) was developed to allow comparison of species and tree component nutrient assimilation and to optimize nutrient management, by quantifying different strategies to manage site nutrients. Biomass, nutrient export and nutrient use efficiency were assessed for three short rotation tree crop species. Nutrient exports following harvest at 3 years of high density (4000 trees ha^?1) were consistently higher in Pinus radiata, with values of 85 kg ha^?1 of N, 11kg ha^?1 of P, and 62 kg ha^?1 of K, than Eucalyptus globulus and Eucalyptus occidentalis. Component NAI was generally in the order of leaf?1 for N in leaves of P. radiata to 4.7 Mg kg^?1 for P in stem‐wood of E. occidentalis, indicating higher sustainability of wood biomass compared with leaf biomass. The leaves for each species contained between 40 and 60% of the total nutrient contents while comprising around 25–30% of the total biomass. These nutrient exports via biomass removal are similar to those that follow 3 years of wheat production in the same region, indicating there is no additional drawdown of nutrient reserves during the tree cropping phase of the rotation.     

Long-term response and recovery to nutrient addition of a partitioned arctic lake

1. To study the bottom‐up linkages in arctic lakes, we treated one side of a partitioned lake with inorganic nitrogen and phosphorus for a 6‐week period each summer for 6 years starting in the summer of 1985. We took a variety of weekly measurements to determine the impact of the nutrient loading on the lake and continued weekly measurements for 2–6 years after the cessation of nutrient loading to observe the recovery of the treated side. The loading rates (2.91 mmol N m^?2 day^?1 and 0.23 mmol P m^?2 day^?1) were five times the calculated loading rates for Toolik Lake, located nearby. 2. In all 6 years of nutrient addition, phytoplankton biomass and productivity were greater in the treated sector than the reference sector. In the first 4 years of nutrient addition there was no flux of phosphorus from the mineral‐rich sediments. This changed in the last 2 years of nutrient addition as phosphorus was released to the lake. 3. The response of the animal community to increased plant production was mixed. One of the four macro‐zooplankton species (Daphnia longiremis) increased in number by about twofold in the first 5 years. However, the copepod Cyclops scutifer showed no response during the treatment phase of the study. The benthic invertebrate response was also mixed. After a 2‐year lag time the snail Lymnaea elodes increased in the treated lake sector but chironomids did not. 4. Ecosystem response to fertilisation was not controlled solely by nutrient addition because phosphorus was not recycled from the sediments until the last 2 years of nutrient addition. Phytoplankton still showed the effects of nutrient addition in the recovery period and the hypolimnion of the treated sector was still anaerobic starting at 6 m in 1996.     

Gravel dredging alters diversity and structure of riverine fish assemblages

1. Human activities affect fish assemblages in a variety of ways. Large‐scale and long‐term disturbances such as in‐stream dredging and mining alter habitat and hydrodynamic characteristics within rivers which can, in turn, alter fish distribution. Habitat heterogeneity is decreased as the natural riffle–pool–run sequences are lost to continuous pools and, as a consequence, lotic species are displaced by lentic species, while generalist and invasive species displace native habitat specialists. Sediment and organic detritus accumulate in deep, dredged reaches and behind dams, disrupting nutrient flow and destroying critical habitat for habitat specialist species. 2. We used standard ecological metrics such as species richness and diversity, as well as stable isotope analysis of δ¹³C and δ¹⁵N, to quantify the differences in fish assemblages sampled by benthic trawls among dredged and undredged sites in the Allegheny River, Pennsylvania, U.S.A. 3. Using mixed‐effects models, we found that total catch, species richness and diversity were negatively correlated with depth (P < 0.05), while species richness, diversity and proportion of species in lithophilic (‘rock‐loving’) reproductive guilds were lower at dredged than at undredged sites (P < 0.05). 4. Principal components analysis and manova revealed that taxa such as darters in brood hider and substratum chooser reproductive guilds were predominantly associated with undredged sites along principal component axis 1 (PC1 and manova P < 0.05), while nest spawners such as catfish and open substratum spawners including suckers were more associated with dredged sites along PC2 (P < 0.05). 5. Stable isotope analysis of δ¹³C and δ¹⁵N revealed shifts from reliance on shallow water and benthic‐derived nutrients at undredged sites to reliance on phytoplankton and terrestrial detritus at deep‐water dredged sites. Relative trophic positions were also lower at dredged sites for many species; loss of benthic nutrient pathways associated with depth and dredging history is hypothesised. 6. The combination of ecological metrics and stable isotope analysis thus shows how anthropogenic habitat loss caused by gravel dredging can decrease benthic fish abundance and diversity, and that species in substratum‐specific reproductive guilds are at particular risk. The effects of dredging also manifest by altering resource use and nutrient pathways within food webs. Management and conservation decisions should therefore consider the protection of relatively shallow areas with suitable substratum for spawning for the protection of native fishes.     

Contrasting responses of phytoplankton and benthic algae to recent nutrient enrichment in Arctic tundra ponds

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The relation between invertebrate drift and two primary controls,discharge and benthic densities,in a large regulated river

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黄河三角洲芦苇湿地底栖无脊椎动物与环境因子的关系研究——以石油开采区与淡水补给区为例

分别于2018年5月和8月对黄河三角洲芦苇湿地19(淡水补给区11处,石油开采区8处)处采样点的底栖无脊椎动物和水体理化指标进行调查采样,运用统计方法分析两个区域物种组成、优势种、多样性、群落结构以及与环境因子的关系。结果表明:两季共采集到底栖无脊椎动物54种,主要以水生昆虫、腹足纲和软甲纲为主,淡水补给区和石油开采区各类群组成差异明显。独立样本T检验表明淡水补给区和石油开采区水体理化指标间差异显著(P0.05)。双因子方差分析显示,昆虫纲和腹足纲密度在两区域差异显著(P0.05),软甲纲和腹足纲密度在季节上差异明显(P0.05)。底栖无脊椎动物优势种共10种,淡水补给区指示物种8种,而石油开采区未发现有指示物种。聚类和非参数多维排序(nM DS)显示,底栖无脊椎动物群落结构相似性较低; RDA结果表明:淡水补给区底栖无脊椎动物群落结构主要受Cond,TDS,Sal,pH,Eh,HCO_3~-,SO_4~(2-)等环境因子的影响。石油开采区底栖无脊椎动物群落结构影响较大的环境因子为HCO_3~-、NH_4-N。     

Natural and man-caused factors affecting the abundance and cycling of dissolved organic substances in precambrian shield lakes

A study on nutrient regeneration processes and a measure of their fluxes at the sediment-water interface was carried out in two different stations of a shallow lagoon of the Po delta river (Italy). A few parameters on the solid fraction (grain-size, porosity, C, N) and pore water profiles of o-P, NH₃, NO _inf3 ^sup– , SiO₂, Tot-CO₂, SO _inf4 ^sup2– , Fe, Mn, Ca, Mg, pH, Eh were determined. At both stations the results were typical for fine sediments rich in organic matter. The ratio of variations of sulphate (SO _inf4 ^sup2– ) to total carbonate demonstrates the main role sulphate reduction plays on the organic matter decay. The use of the ratios of variations of sulphate (SO _inf4 ^sup2– ) to ammonia (NH₃) and of sulphate (SO _inf4 ^sup2– ) to phosphate (o-P) in pore waters enabled us to calculate the C/N/P of the decomposing organic matter. Obtained C/N/P indicated an enrichment of N and P with regard to C/N/P ratios of the solid fraction, due to the selective stripping of N and P during organic matter mineralization. This phenomenon decreases with depth, where organic matter becomes more refractory. Calculations on saturation degrees have shown the possibility of authigenic calcite, apatite and rhodochrosite precipitation in sediments. Nutrient fluxes were estimated for SiO₂, NH₃ and o-P by means of benthic chambers and modelling the pore water profiles. The model used for the calculation of fluxes allowed us to account for the bioturbation-irrigation influence near the interface, by means of a cumulative diffusion coefficient. Directly measured fluxes proved to be always significantly greater than the theoretical ones. These differences seem to be due to surface processes which do not affect pore water concentrations (degradation of fresh materials at the interface; micro-bioturbation by small gasteropoda such as Hydrobia ventrosa) and/or to the different concept of the two methods in time and space. Number, size and biomass of macrobenthic species living in the sediment underneath the benthic chambers were determined. The comparison between data on macrobenthic populations and flux values showed a good relationship between the number of organisms and benthic fluxes within each station. However, flux variations between stations are to be attributed mainly to the different arrangement of the tubes of the polychaetes Polydora ciliata in the sediment.