Eutrophic Systems Research Articles

Heterotrophic bacterio‐plankton in thawed lakes of the northern part of Western Siberia controls the CO2 flux to the atmosphere

This work reports on results of bacterio‐plankton characterisation in thaw lakes of the northern part of Western Siberia via measurement the number of various groups of heterotrophic bacteria and the intensity of primary production/respiration in the water column. The eutrophic systems at the beginning of lake formation (permafrost thawing) are being replaced by essentially oligotrophic systems at the final, mature stage of the lake development (khasyrey). The CO2 flux from the lake surface to the atmosphere associated with microbial degradation of organic matter (107 ± 50 t C/km2/y) is at least one order of magnitude higher than the riverine organic carbon flux. The future climate evolution in this region, consisting in rising ground temperature and precipitation increase will bring about further acceleration of dissolved organic matter degradation in the water column and amplification of CO2 release to the atmosphere.

Occurrence of microcystin-producing cyanobacteria in Ugandan freshwater habitats.

Microcystins (MCs) are cyclic heptapeptides, which are the most abundant toxins produced by cyanobacteria in freshwater. The phytoplankton of many freshwater lakes in Eastern Africa is dominated by cyanobacteria. Less is known, however, on the occurrence of MC producers and the production of MCs. Twelve Ugandan freshwater habitats ranging from mesotrophic to hypertrophic conditions were sampled in May and June of 2004 and April of 2008 and were analyzed for their physicochemical parameters, phytoplankton composition, and MC concentrations. Among the group of the potential MC-producing cyanobacteria, Anabaena (0-10(7) cells ml(-1)) and Microcystis (10(3)-10(7) cells ml(-1)) occurred most frequently and dominated in eutrophic systems. A significant linear relationship (n = 31, r(2) = 0.38, P < 0.001) between the Microcystis cell numbers and MC concentration (1.3-93 fg of MC cell(-1)) was observed. Besides [MeAsp(3), Mdha(7)]-MC-RR, two new MCs, [Asp(3)]-MC-RY and [MeAsp(3)]-MC-RY, were isolated and their constitution was assigned by LC-MS(2). To identify the MC-producing organism in the water samples, (i) the conserved aminotransferase domain part of the mcyE gene that is indicative of MC production was amplified by general primers and cloned and sequenced, and (ii) genus-specific primers were used to amplify the mcyE gene of the genera Microcystis, Anabaena, and Planktothrix. Only mcyE genotypes that are indicative of Microcystis sp. were obtained via the environmental cloning approach (337 bp, 96.1-96.7% similarity to the Microcystis aeruginosa strain PCC7806). Accordingly, only the mcyE primers, which are specific for Microcystis, revealed PCR products. We concluded that Microcystis is the major MC-producer in Ugandan freshwater.

Bioturbation potential of chironomid larvae for the sediment–water phosphorus exchange in simulated pond systems of varied nutrient enrichment

Chironomid larvae (2.0 individuals/cm 2) were introduced in sediment–water microcosms of 3.0 l capacity to assess the impact of bioturbation on phosphorus flux across sediment–water interface, under different nutrient-enriched conditions. Recruitment of chironomid resulted in 21% and 19% increase in aquatic orthophosphate and nitrate quanta, respectively, with concomitant decrease in nutrient concentration in the sediment compared to macrofauna-free controls under mesotrophic condition. It implied that cost of fertilizer for biological production could be curtailed by at least 19–21% by recovering nutrients stored in the sediment pool. Bioturbation-induced orthophosphate flux under chironomid impacted mesotrophic treatment was 2.3- and 1.8-fold greater than that under bioturbated eutrophic treatment, suggesting that the macrofaunal impact was reduced in the presence of higher nutrient load perhaps due to physicochemical stressors under eutrophic condition. Nevertheless, chironomid larvae can further accelerate nutrient enrichment in the eutrophic system that may invite a “snow ball effect” towards a hypereutrophic one. The counts of both heterotrophic and phosphate solubilizing bacteria show strong positive correlation with orthophosphate concentration in water and the correlation also exists between organic carbon concentration in sediment and phosphate in overlying water. This implied that the accelerated phosphate flux was the result of coordinated eco-engineering activities of chironomid larvae and microbe-mediated mineralization of organic matter.

Use of an oxygen-releasing compound to aerate eutrophic reservoir water

This study examined the use of an Oxygen Release Compound (ORC®) as a slow release chemical aeration agent within eutrophic surface water systems. Bench and pilot-scale experiments involving ORC® treatment of eutrophic reservoir water were conducted to determine operational properties of ORC® and examine its performance as an aeration agent within a surface water environment. The bench-scale study involved the application of 10, 25 and 50 g ORC® doses to 1 L Erlenmeyer flasks containing various water and sediment samples. The pilot-scale study involved a scaled up simulation of full-scale reservoir systems using large fiberglass tanks, where water quality parameters in an ORC®-treated tank were compared to an untreated tank over a 4 month period. The results of these experiments indicate that the application of ORC® can result in substantial increases in dissolved oxygen and pH when applied to deoxygenated water systems. Within the pilot scale study, a 300 g/m2 dose of ORC® at the sediment water interface prevented the onset of anoxic conditions over a 4 month growing season period, releasing approximately 20% of its mass as oxygen within this time frame.

Effects of agricultural subsidies of nutrients and detritus on fish and plankton of shallow‐reservoir ecosystems

Agricultural activities increase exports of nutrients and sediments to lakes, with multiple potential impacts on recipient ecosystems. Nutrient inputs enhance phytoplankton and upper trophic levels, and sediment inputs can shade phytoplankton, interfere with feeding of consumers, and degrade benthic habitats. Allochthonous sediments are also a potential food source for detritivores, as is sedimenting autochthonous phytodetritus, the production of which is stimulated by nutrient inputs. We examined effects of allochthonous nutrient and sediment subsidies on fish and plankton, with special emphasis on gizzard shad (Dorosoma cepedianum). This widespread and abundant omnivorous fish has many impacts on reservoir ecosystems, including negative effects on water quality via nutrient cycling and on fisheries via competition with sportfish. Gizzard shad are most abundant in agriculturally impacted, eutrophic systems; thus, agricultural subsidies may affect reservoir food webs directly and by enhancing gizzard shad biomass. We simulated agricultural subsidies of nutrients and sediment detritus by manipulating dissolved nutrients and allochthonous detritus in a 2 x 2 factorial design in experimental ponds. Addition of nutrients alone increased primary production and biomass of zooplanktivorous fish (bluegill and young-of-year gizzard shad). Addition of allochthonous sediments alone increased algal sedimentation and decreased seston and sediment C:P ratios. Ponds receiving both nutrients and sediments showed highest levels of phytoplankton and total phosphorus. Adult and juvenile gizzard shad biomass was enhanced equally by nutrient or sediment addition, probably because this apparently P-limited detritivore ingested similar amounts of P in all subsidy treatments. Nutrient excretion rates of gizzard shad were higher in ponds with nutrient additions, where sediments were composed mainly of phytodetritus. Therefore, gizzard shad can magnify the direct effects of nutrient subsidies on phytoplankton production, and these multiple effects must be considered in strategies to manage eutrophication and fisheries in warmwater reservoir lakes where gizzard shad can dominate fish biomass.

Testate amoebae as indicators of 20th century environmental change in Lake Zhangdu, China

We investigated the response of lacustrine testate amoeba communities to 20 th century environmental changes in Lake Zhangdu, China. The lake and surrounding region have undergone dramatic changes in the past century, including the expansion of agricultural and industrial activities, and associated hydrological modifications that led to the isolation of the lake from Yangtze River. We assessed the potential effects of these activities on testate amoeba communities of the lake by examining subfossil assemblages in contiguous samples along a 35-cm long, 210 Pb dated sediment core. A total of 25 testate amoeba taxa belonging to 4 genera (Difflugia, Centropyxis, Neztelia, and Pentagonia) were encountered in the core. The largest change in the composition of testate amoeba communities occurred in the 1960s, coincident with the expansion of human activities in the watershed. Testate amoeba communities shifted from species characteristic of relatively oligotrophic lakes (e.g. Difflugia biwae, D. tuberspinifera and D. pristis) to species more common to mesotrophic and eutrophic systems (e.g. Difflugia oblonga, D. corona, D. smilion and D. lanceolata). Our results provide valuable baseline data on testate amoebae and water conditions before and after major 20 th century human impact on the lake ecosystem. Lake Zhangdu was recently reconnected with the Yangtze River as part of ongoing conservation efforts, and future monitoring of testate amoeba communities could be used to inform management and assess restoration success.

Effects of microzooplankton growth and trophic interactions on herbivory in coastal and offshore environments

We performed serial dilution experiments to estimate rates of gross phytoplankton growth (μ) and grazing mortality (m) in both eutrophic (Corpus Christi Bay, Texas, USA) and olig- otrophic (offshore Gulf of Mexico) waters. Two parallel experiments were performed in both environ- ments, with seawater pre-screened through 153 or 25 µm mesh to observe the responses of microzoo- plankton (MZP) to dilution treatments. MZP biomass changed over the duration of the experimental incubations; in several treatments, MZP net growth rates were >1 d -1 . Patterns of growth varied between dilutions and initial screening size. In the eutrophic system, the ratio of phytoplankton graz- ing mortality rate to gross phytoplankton growth rate (m/μ) was 1.10 ± 0.54 (mean ± SD) versus 0.41 ± 0.65 when screened through 153 and 25 µm mesh, respectively. This difference was attributed to cas- cading trophic interactions among MZP size groups leading to suppression of the primary herbivores in the 25 µm fraction and, in turn, a lower value of m. A food web model consisting of multiple trophic levels was constructed to examine the role of MZP growth and trophic interactions on measurements of μ and m. The model, using 3 interacting groups of MZP, was able to reproduce experimental results. Model simulations demonstrated that MZP growth during incubation leads to an overestima- tion of m. Non-linearity in the phytoplankton growth response curves was due to MZP growth and trophic interactions in these model simulations, as variable feeding responses were not incorporated into the models. Trophic interactions among MZP can provide context to measurements of μ and m and insight into microbial food web efficiency.

Quantity and bioavailability of sediment organic matter as signatures of benthic trophic status

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 375:41-52 (2009) - DOI: https://doi.org/10.3354/meps07735 Quantity and bioavailability of sediment organic matter as signatures of benthic trophic status Antonio Pusceddu1,*, Antonio Dell’Anno1, Mauro Fabiano2, Roberto Danovaro1 1Department of Marine Sciences, Polytechnic University of Marche, Via Brecce Bianche, 60131 Ancona, Italy 2Department for the Study of the Territory and its Resources, University of Genoa, Corso Europa, 26, 16100 Genova, Italy *Email: a.pusceddu@univpm.it ABSTRACT: Tools used for assessing marine trophic status are generally based on water column characteristics, which, however, may provide unreliable classification of the benthic trophic status. Here, we provide evidence from the literature that quantity and bioavailability of sediment organic matter are reliable proxies to assess benthic marine trophic status. We compiled data on the protein, carbohydrate and lipid concentration of sediments from different oceanic and coastal regions and varying water depths. The concentration of these 3 components as a whole (biopolymeric carbon) was found to be significantly correlated (r = 0.84) with the total organic carbon concentration, suggesting that the biopolymeric fraction is representative of the total organic carbon pool. However, the systematic variation of the biopolymeric fraction was higher than that of total organic carbon concentrations, suggesting that biopolymeric carbon is a more sensitive proxy of benthic trophic status than is the total carbon pool. Furthermore, biopolymeric carbon was significantly correlated to the amount of phytopigments, indicating that biopolymeric carbon accumulation in the sediment is related to inputs of algal carbon. Biopolymeric carbon concentrations were also positively correlated to the sediment community oxygen consumption, suggesting that the progressive accumulation of biopolymeric carbon could be an additional co-factor potentially responsible for hypoxic or anoxic events. The enzymatically digestible and algal fractions of biopolymeric carbon decreased in sediments with increasing biopolymeric carbon content (i.e. eutrophic systems), suggesting that organic carbon in eutrophic sediments is mostly refractory in nature. We propose that a biopolymeric carbon concentration in the sediment of >2.5 mg C g–1, being associated with a bioavailable fraction of <10%, can be considered as a threshold level at which benthic consumers may experience mostly refractory organic carbon. KEY WORDS: Marine sediments · Organic matter · Trophic status Full text in pdf format PreviousNextCite this article as: Pusceddu A, Dell’Anno A, Fabiano M, Danovaro R (2009) Quantity and bioavailability of sediment organic matter as signatures of benthic trophic status. Mar Ecol Prog Ser 375:41-52. https://doi.org/10.3354/meps07735 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 375. Online publication date: January 26, 2009 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2009 Inter-Research.

Animal-sediment relationships: Evaluating the ‘Pearson–Rosenberg paradigm’ in Mediterranean coastal lagoons

We investigated the applicability of the Pearson–Rosenberg (P–R) conceptual model describing a generalized pattern of response of benthic communities in relation to organic enrichment to Mediterranean Sea coastal lagoons. Consistent with P–R model predictions, benthic diversity and abundance showed two different peaks at low (>2.5–5 mg g −1) and high (>25–30 mg g −1) total organic carbon (TOC) ranges, respectively. We identified TOC thresholds indicating that risks of reduced benthic diversity should be relatively low at TOC values < about 10 mg g −1, high at TOC values > about 28 mg g −1, and intermediate at values in-between. Predictive ability within these ranges was high based on results of re-sampling simulation. While not a direct measure of causality, it is anticipated that these TOC thresholds should serve as a general screening-level indicator for evaluating the likelihood of reduced sediment quality and associated bioeffects in such eutrophic systems of the Mediterranean Sea.

Aplicação de Phoslock® para Remoção de Fósforo e Controle de Cianobactérias Tóxicas

This article describes the potential of Phoslock® to adsorb phosphorous in water bodies and, consequently, to reduce the occurrence of cyanobacteria blooms in eutrophic systems. This product was developed recently in Australia, to restore both freshwater and marine ecosystems, and it is environmentally harmless and functional within a wide gradient of pH (4-11). Experimental tests were performed in the laboratory in order to test product efficiency in removing filterable reactive phosphorous (FRP) in solutions with concentrations of 0.5 and 1.0 mg L-1. Moreover, we tested the potential effect of Phoslock in reducing the growth of a toxic strain of cyanobacteria, Microcystis aeruginosa (NPLJ4). The treatments with Phoslock, have shown significant immobilization (p 94%, p<0,001). The results have corroborated Phoslock’s ability to remove sources of phosphorous from aquatic systems and to limit cyanobacteria growth due to the lack of this nutrient. Among the existing alternatives to control eutrophication, Phoslock is certainly innovative and immobilizes phosphates within the ecosystem without promoting side effects on aquatic organisms and human beings

Increases in phosphorus at the sediment-water interface may influence the initiation of cyanobacterial blooms in an oligotrophic lake

Freshwater cyanobacteria have been implicated as both indicators and agents of ecosystem change (Wetzel 2001). Gloeotrichia echinulata is a nuisance cyanobacterium common in mesoto eutrophic systems that has recently begun to bloom in oligoto mesotrophic lakes across northern New England, USA (Carey et al. 2008). Between 2002 and 2006, at least 27 outbreaks of G. echinulata were confirmed in oligotrophic lakes across Maine and New Hampshire that do not normally experience algal blooms. These blooms are of particular concern because G. echinulata produces the toxin microcystin-LR (Carey et al. 2007) and can transport a considerable amount of phosphorus (P) from the sediment into the water column (Istvanovics et al. 1993). At present, little is known about the mechanisms that have stimulated G. echinulata blooms in oligotrophic systems. We hypothesized that increased P concentrations in lake sediments may be responsible, in part, for G. echinulata blooms in low-nutrient lakes. Many oligotrophic lakes have low concentrations of P in their water, but high concentrations of P in their sediments (Maassen et al. 2005), potentially due to increasing external P loads (Wetzel 2001). Interestingly, many of the lakes in Maine that are now experiencing G. echinulata blooms had been classified as “at risk from new watershed development” (Maine Department of Environmental Services, unpublished), indicating that those lakes may be subject to increased sediment P. Thus, one hypothesis is that the recent G. echinulata blooms may be occurring in response to increased P concentrations in lake sediment: “eutrophied” sediment in an oligotrophic lake. Phosphorus plays a major role in G. echinulata’s complex life cycle. Overwintering akinetes, or resting cells, must absorb a substantial amount of phosphate from the sediment pore water during a germination and growth phase on the lake sediment before recruitment into the water column (Pettersson et al. 1993, Karlsson 2003, Karlsson-Elfgren et al. 2004), which takes ~2.5–4 weeks after the initiation of germination. Phosphorus absorbed in the sediments fuels planktonic cell division; G. echinulata’s P uptake in the water column is thought to be negligible (Istvanovics et al. 1993, Tymowski & Duthie 2000). Because of the large pool of sediment P that exists, even in oligotrophic lakes (Maassen et al. 2005), and G. echinulata’s apparent demand for P from the sediment, we hypothesized that akinetes would exhibit increased germination and recruitment when exposed to increased concentrations of sediment P. We used an in situ nutrient-diffusing substrate experiment to test this hypothesis in an oligotrophic lake while simultaneously monitoring G. echinulata recruitment and P at the sediment-water interface.

Phytoplankton biodiversity changes in a shallow tropical reservoir during the hypertrophication process

Study aimed at evaluating phytoplankton biodiversity changes in a shallow tropical reservoir during its hypertrophication process. Samplings were carried out monthly during 8 consecutive years (1997-2004) in 5 depths. Conspicuous limnological changes in the reservoir derived from the presence and/or removal of the water hyacinth, characterized 3 different phases. Over the time series, reservoir changed from a typical polymictic eutrophic system to hypertrophic one, leading to a reduction of approximately 70 species (average 37%). Chlorophyceae accounted for the highest species richness (46%) among all algal classes and strictly followed total species richness variation. Internal feedback mechanisms intensification over phase III clearly promoted the sharp decrease in biodiversity. Highest decreases, mainly during springs, occurred simultaneously to the highest Cyanobacteria blooms. Increased turbidity due to heavy phytoplankton blooms suppressed all other algal groups, so that at the end of the present study even Cyanobacteria species richness decreased. Total dissolved phosphorous was included in most of the best selected models used to analyze the temporal patterns in species richness loss. Present data show that biodiversity loss following trophic change was not a single dimension of a single factor but, rather, a template of factors (e.g. light, stability) co-varying in consequence of the larger levels of biomass supported in the reservoir.

Does reservoir trophic status influence the feeding and growth of the sharptooth catfish, Clarias gariepinus (Teleostei: Clariidae)?

The diet and growth of sharptooth catfish, Clarias gariepinus, in an oligotrophic system (Kat River Reservoir, Eastern Cape, South Africa) were compared to those in a eutrophic system (Laing Reservoir, Eastern Cape) to determine if the trophic status of a waterbody had an effect on the growth rate of the species. In order of importance, the diet of catfish in Kat River Reservoir consisted of fish, insects, zooplankton, plant material and other items, while the diet of catfish in Laing Reservoir consisted of fish, plant material, zooplankton, other vertebrates and insects. The diets of catfish in the two reservoirs had a similarity index of 68.1% and there was no significant difference in their nutritional value. Fish prey was the most important dietary component in both reservoirs. Temperature regime and zooplankton and zoobenthos density were similar in both systems. However, fish prey density was significantly higher in the eutrophic Laing Reservoir and catfish grew significantly faster in that system. The slower growth rate in Kat River Reservoir was attributed to the higher energy costs associated with the capture of fish prey, which was less abundant than in Laing Reservoir. Trophic status therefore had an indirect effect on catfish growth by influencing the availability of fish prey.

Living in highly dynamic polluted river floodplains, do contaminants contribute to population and community effects?

The aim of this paper was to collect evidence for the effects of contaminants on biota in a highly dynamic river Rhine floodplain. To this purpose we reviewed the results of circa 10 studies performed in this floodplain. The floodplain was contaminated with elevated levels of cadmium, copper, PAHs, and PCBs and high levels of zinc which were at some sites above legislative values. The results showed that the present contaminants were accumulated by the floodplain inhabiting organisms, but meanwhile population and community effects were ambiguous. Only for the mayfly Ephoron virgo clear effects were detected at the level of the single floodplain. The absence of clear population and community effects is puzzling since at lower contaminant concentrations adverse effects were detected in other environments. Factors that may mask toxic effects include flooding and food quality and quantity. We conclude that given the site specific conditions, being an open, eutrophic system with a highly dynamic flooding pattern, assessment of the contribution of toxicants to observed population density or biomass and community composition requires 1] an increase in number of replicates; 2] a larger scale of investigation and 3] comparison to stable systems with comparable contamination levels.

Gloeotrichia echinulata blooms in an oligotrophic lake: helpful insights from eutrophic lakes

Nuisance algal blooms are widely reported from eutrophic and hypereutrophic systems, but are relatively rare in oligotrophic systems. Recently, the cyanobacterium Gloeotrichia echinulata has begun blooming in oligotrophic to mesotrophic lakes in the northeastern USA. We explored whether existing knowledge about G. echinulata, derived from eutrophic systems, would be useful in predicting the recruitment dynamics of G. echinulata in an oligotrophic lake in central New Hampshire, USA. Our results indicate that, as in eutrophic lakes, there was a seasonal peak in both recruitment and surface abundance, with colonies recruiting from shallow (2 m) but not deep (≥5 m) sediments. Recruitment provided approximately 20-45% of the colonies to the 2005 G. echinulata surface bloom, within the range reported for eutrophic lakes. Thus, we conclude that observations of G. echinulata in eutrophic systems may be useful in predicting G. echinulata recruitment dynamics in oligotrophic lakes. Moreover, recruitment from the sediments appears to be an important mechanism that allows G. echinulata to bloom in low- as well as high-nutrient lakes. This benthic-pelagic coupling may, over time, result in an important transfer of nutrients in such oligotrophic lakes.

Species richness and cell-size distribution: size structure of phytoplankton communities

If the size dependence of species richness varies across ecosystems, it should be reflected in the size distribution of total abundance. Using a database of phytoplankton abundance, species composition and cell size from coastal, shelf and open-ocean environments, we show that the biogeographical patterns of phytoplankton size distribution in the ocean are a result of systematic changes in the relationship between species richness and cell size. Our results indicate that, regard- less of the environmental conditions, population abundance decreases consistently to the -3/4 power of cell size. By contrast, marine phytoplankton diversity peaks at small sizes in oligotrophic waters but is either a log-normal function or independent of cell size in eutrophic systems. It is argued that, operating on evolutionary time scales, size-dependent biophysical constraints for resource acquisi- tion are reflected in the size distribution of species richness and consequently in the size structure of phytoplankton communities in the ocean. These findings indicate that the way in which biological diversity changes with body size is crucial to a better understanding of the structure and functioning of microbial plankton communities and how energy flows through pelagic ecosystems.

Implications of seston settling on phosphorus dynamics in three reservoirs of contrasting trophic state

We performed a comparative study in three reservoirs based on field measurements and laboratory experiments determine the impact of seston sedimentation on phosphorus dynamics. While we found a strong relationship between the trophic state and the settling flux of dry material in the study reservoirs, an increase in the trophic state did not lead to an increase in the export ratio (fraction of the primary production lost by sinking). In laboratory experiments, the analysis of biotic and abiotic contributions to phosphate (PO 4 3- ) regeneration/uptake by the sinking particles showed that an increase in the trophic state led to a significant vertical segregation in the observed patterns. Settled matter collected in both upper and lower sediment traps exhibited similar temporal patterns in the oligotrophic reservoir (Quentar), while clear spatial differences occurred in the more eutrophic systems (Cubillas and Beznar) as result of the vertical distribution of PO 4 3- characterizing warm monomictic eutrophic systems during summer thermal stratification. Chemical analysis of the settling matter in the oligotrophic reservoir (Quentar) showed that the sinking particles became richer in phosphorus during sedimentation, especially in the organic P pools. In contrast, P content in settling matter decreased with depth in the hypertrophic reservoir (Beznar), especially the org-P at the thermocline. This indicates that the mineralization of settling organic matter is important for maintaining high concentration of PO 4 3- in the deeper layers. However, no significant vertical differences in P content in settled matter were detected during the whole study period.

A Review of Nontraditional Biomanipulation

The aim of this review is to identify problems, find general patterns, and extract recommendations for successful management using nontraditional biomanipulation to improve water quality. There are many obstacles that prevent traditional biomanipulation from achieving expectations: expending largely to remove planktivorous fish, reduction of external and internal phosphorus, and macrophyte re-establishment. Grazing pressure from large zooplankton is decoupled in hypereutrophic waters where cyanobacterial blooms flourish. The original idea of biomanipulation (increased zooplankton grazing rate as a tool for controlling nuisance algae) is not the only means of controlling nuisance algae via biotic manipulations. Stocking phytoplanktivorous fish may be considered to be a nontraditional method; however, it can be an effective management tool to control nuisance algal blooms in tropical lakes that are highly productive and unmanageable to reduce nutrient concentrations to low levels.Although small enclosures increase spatial overlap between predators and prey, leading to overestimates of the impact of predation, microcosm and whole-lake experiments have revealed similar community responses to major factors that regulate lake communities, such as nutrients and planktivorous fish. Both enclosure experiments and large-scale observations revealed that the initial phytoplankton community composition greatly impacted the success of biomanipulation. Long-term observations in Lake Donghu and Lake Qiandaohu have documented that silver carp (Hypophthalmichthys molitrix) and bighead carp (H. nobilis) (two filter-feeding planktivorous species commonly used in management) can suppress Microcystis blooms efficiently. The introduction of silver and bighead carp could be an effective management technique in eutrophic systems that lack macrozooplankton. We confirmed that nontraditional biomanipulation is only appropriate if the primary aim is to reduce nuisance blooms of large algal species, which cannot be controlled effectively by large herbivorous zooplankton. Alternatively, this type of biomanipulation did not work efficiently in less eutrophic systems where nanophytoplankton dominated.

Nitrous oxide distribution and its origin in the central and eastern South Pacific Subtropical Gyre

Abstract. The mechanisms of microbial nitrous oxide (N2O) production in the ocean have been the subject of many discussions in recent years. New isotopomeric tools can further refine our knowledge of N2O sources in natural environments. This study compares hydrographic, N2O concentration, and N2O isotopic and isotopomeric data from three stations along a coast-perpendicular transect in the South Pacific Ocean, extending from the center (Sts. GYR and EGY) of the subtropical oligotrophic gyre (~26° S; 114° W) to the upwelling zone (St. UPX) off the central Chilean coast (~34° S). Although AOU/N2O and NO3− trends support the idea that most of the N2O (mainly from intermediate water (200–600 m)) comes from nitrification, N2O isotopomeric composition (intramolecular distribution of 15N isotopes) expressed as SP (site preference of 15N) shows low values (10 to 12\\permil) that could be attributed to the production through of microbial nitrifier denitrification (reduction of nitrite to N2O mediated by ammonium oxidizers). The coincidence of this SP signal with high – stability layer, where sinking organic particles can accumulate, suggests that N2O could be produced by nitrifier denitrification inside particles. It is postulated that deceleration of particles in the pycnocline can modify the advection - diffusion balance inside particles, allowing the accumulation of nitrite and O2 depletion suitable for nitrifier denitrication. As lateral advection seems to be relatively insignificant in the gyre, in situ nitrifier denitrification could account for 40–50% of the N2O produced in this layer. In contrast, coastal upwelling system is characterized by O2 deficient condition and some N deficit in a eutrophic system. Here, N2O accumulates up to 480% saturation, and isotopic and isotopomer signals show highly complex N2O production processes, which presumably reflect both the effect of nitrification and denitrification at low O2 levels on N2O production, but net N2O consumption by denitrification was not observed.

Tuning the tide: creating ecological conditions for tidal marsh development in a flood control area

The Schelde estuary, characterised as a turbid, polluted and eutrophic system, has nowadays reached a turning point in the restoration of its water quality. During the past century, human activities have reduced the intertidal areas, essential in the estuarine ecosystem for nutrient cycling and the self-cleaning capacity. Today, in combination with a master plan to protect the population from storm surges, an opportunity rises to restore areas with a tidal influence. One specific option of combining safety and ecology is the creation of flood control areas (FCA) under the influence of a controlled reduced tide (CRT). These specific areas will differ in many ways from fully tidal areas. However, these areas can fulfill important ecological functions with effects on aeration, nitrification, denitrification, sedimentation and primary production in the estuary. Opportunities for ecological development within a CRT have been investigated for a specific case. The ecology within a CRT showed to be very case specific, depending e.g. on the morphology of the area, the sluice design and the local water quality. Depending on the sluice design, water quality can be improved and sedimentation can be influenced. Possible measures to design a CRT with a rich habitat variation are discussed.