Limnology

Two new species of Nitzschia (Bacillariophyta) from shallow wetlands of Peninsular India

by Karthick Balasubramanian

The majority of species belonging to the genus Nitzschia are distinguished by minute taxonomic features that are... more The majority of species belonging to the genus Nitzschia are distinguished by minute taxonomic features that are difficult to observe and document. Currently, geographical distributions for many species are recognized as cosmopolitan; in contrast endemic species are poorly documented and studied. Our study describes two new species of Nitzschia from shallow wetlands across the Bangalore urban district of peninsular India, Nitzschia taylorii, sp. nov. and Nitzschia williamsi, sp. nov. Morphological analyses of these new species were performed with light and scanning electron microscopy, and the ecology of inhabited wetlands are discussed briefly. New species records from urban polluted wetlands provide evidence for broadening taxonomic and ecological investigations of cosmopolitan genera like Nitzschia in the Southern Hemisphere.

Hydrogeochemistry and rare earth element behavior in a volcanically acidified watershed in Patagonia, Argentina

by Christopher Shope

Two new species of Nitzschia (Bacillariophyta) from shallow wetlands of Peninsular India

by Alakananda B

ALAKANANDA, B. MAHESH, M.K., PAUL B. HAMILTON, G. SUPRIYA, B. KARTHICK
& T. V. RAMACHANDRA,

Measuring submerged macrophyte standing crop in shallow rivers: a test of methodology

by Kevin Wood

Wood, K.A., Stillman, R.A., Clarke, R.T., Daunt, F. & O'Hare, M.T. (in press). Measuring submerged macrophyte standing crop in shallow rivers: a test of methodology. Aquatic Botany

High natural variability in space and time can make accurate measurements of macrophyte standing crop difficult.... more High natural variability in space and time can make accurate measurements of macrophyte standing crop difficult. Accuracy of such measurements could be improved by quantifying the relationships between the different methods of measuring standing crop which are available to researchers. In this study we compare cover, volume, and biomass as measures of standing crop. Percentage cover, percentage volume, and dry weight biomass estimates were positively related (R2(adj) range = 54–96%), but these relationships were significantly different between sites, and to a lesser extent between months. Biomass was related (R2(adj) range = 18–73%) to stand height. Furthermore, cover, volume and biomass indicated different seasonal trends in standing crop at the two study sites. Our study presents a suite of standing crop measures that exhibit close congruence, can be measured efficiently and minimise destructive sampling in situ, attributes which will aid in the design and implementation of future macrophyte measurement protocols for shallow rivers.

River eutrophication: Irrigated vs. non-irrigated agriculture through different spatial scales

by Laura Monteagudo

Co-authored with Jose Luis Moreno and Felix Picazo
Doi:10.1016/j.watres.2012.02.035

The main objective of this study was to determine how spatial scale may affect the results when relating land use to... more The main objective of this study was to determine how spatial scale may affect the results when relating land use to nutrient enrichment of rivers and, secondly, to investigate which agricultural practices are more responsible for river eutrophication in the study area. Agriculture was split into three subclasses (irrigated, non-irrigated and low-impact agriculture) which were correlated to stream nutrient concentration on four spatial scales: large scale (drainage area of total subcatchment and 100 m wide subcatchment corridors) and local scale (5 and 1 km radius buffers). Nitrate, ammonium and orthophosphate concentrations and land use composition (agriculture, urban and forest) were measured at 130 river reaches in south-central Spain during the 2001–2009 period. Results suggested that different spatial scales may lead to different conclusions. Spatial autocorrelation and the inadequate representation of some land uses produced unreal results on large scales. Conversely, local scales did not show data autocorrelation and agriculture subclasses were well represented. The local scale of 1 km buffer was the most appropriate to detect river eutrophication in central Spanish rivers, with irrigated cropland as the main cause of river pollution by nitrate. As regards river management, a threshold of 50% irrigated cropland within a 1 km radius buffer has been obtained using breakpoint regression analysis. This means that no more than 50% of irrigation croplands should be allowed near river banks in order to avoid river eutrophication. Finally, a methodological approach is proposed to choose the appropriate spatial scale when studying river eutrophication caused by diffuse pollution like agriculture.

Population-Level Metrics of Trophic Structure Based on Stable Isotopes and Their Application to Invasion Ecology

by Jonathan Grey

PLoS One 2012

Biological invasions are a significant driver of human-induced global change and many ecosystems sustain sympatric... more Biological invasions are a significant driver of human-induced global change and many ecosystems sustain sympatric invaders. Interactions occurring among these invaders have important implications for ecosystem structure and functioning, yet they are poorly understood. Here we apply newly developed metrics derived from stable isotope data to provide quantitative measures of trophic diversity within populations or species. We then use these to test the hypothesis that sympatric invaders belonging to the same functional feeding group occupy a smaller isotopic niche than their allopatric counterparts. Two introduced, globally important, benthic omnivores, Louisiana swamp crayfish (Procambarus clarkii) and carp (Cyprinus carpio), are sympatric in Lake Naivasha, Kenya. We applied our metrics to an 8-year data set encompassing the establishment of carp in the lake. We found a strong asymmetric interaction between the two invasive populations, as indicated by inverse correlations between carp abundance and measures of crayfish trophic diversity. Lack of isotopic niche overlap between carp and crayfish in the majority of years indicated a predominantly indirect interaction. We suggest that carp-induced habitat alteration reduced the diversity of crayfish prey, resulting in a reduction in the dietary niche of crayfish. Stable isotopes provide an integrated signal of diet over space and time, offering an appropriate scale for the study of population niches, but few isotope studies have retained the often insightful information revealed by variability among individuals in isotope values. Our population metrics incorporate such variation, are robust to the vagaries of sample size and are a useful additional tool to reveal subtle dietary interactions among species. Although we have demonstrated their applicability specifically using a detailed temporal dataset of species invasion in a lake, they have a wide array of potential ecological applications.

Migratory benthic fishes may induce regime shifts in a tropical floodplain pond

by Roger Paulo Mormul

Freshwater Biology 2012 - accepted and awaiting online first version

Alternative states are a widely recorded phenomenon in shallow lakes, which may shift between turbid and clear water... more Alternative states are a widely recorded phenomenon in shallow lakes, which may shift between turbid and clear water conditions. Here we investigate whether such shifts in a tropical floodplain pond may be related to the effect of the flood pulse regime on the community structures of fish and macrophytes. Using a long-term data set, we demonstrate how benthic fish migration together with colonization by submerged plants affected the transition from a turbid to a macrophyte-dominated state in a floodplain pond without top-down control. In our study, the turbid state occurred mostly during low water phases (in contrast to the pattern in temperate regions) and was largely characterized by high values for the biomass of benthic fish, chlorophyll-a and total phosphorous. During the period of rising water levels, the migration of benthic fish out of the pond occurs simultaneously with the establishment of submerged plants, while water turbidity decreases along with phytoplankton and nutrient concentrations, inducing a clear-water phase. However, when submerged plants are absent and fish migration is low, a transient state is generated. We suggest that, different from the temperate ponds and shallow lakes, where the main driving mechanisms establishing alternative states are related to cascading effects via the food chain, in the tropical ponds and shallow lakes, resuspension of sediments by benthic fish may play the most significant role in establishing alternative states. However, the effect of the flood pulse regime plays an important role in the temporal dynamics of fish community structure by controlling benthic fish migration.

Water brownification may increase the invasibility of a submerged non-native macrophyte

by Roger Paulo Mormul

Biological Invasions 2012 - online first

Environmental conditions and human activities play a significant role in structuring novel assemblages of native and... more Environmental conditions and human activities play a significant role in structuring novel assemblages of native and non-native species. Ongoing and future climatic change may alter the performance of native and non-native species and their biotic interactions. In the northern hemisphere, expected climate changes include warmer temperatures and higher precipitation, the latter of which may increase dissolved organic carbon (humic) concentrations, resulting in browner water in aquatic ecosystems (brownification). We tested the effects of elevated temperature (3ºC) and brownification on native and non-native aquatic plant production in mesocosms over 56 days. Elodea canadensis, an aquatic invasive plant, had higher relative growth rate in terms of both length and weight, as well as higher weight to length ratio when grown in brown vs. clear water; E. canadensis did not respond to temperature treatments. Different functional groups of native producers (phytoplankton, periphyton, macrophytes) showed different relationships to temperature and brownification treatments, with the macrophyte response being most notable because it was opposite to that of E. canadensis. Native macrophytes decreased in biomass in browner water, where they represented about 40% of total biomass compared to 85% in clear water. In regression analyses, E. canadensis length RGR was best predicted only by water color treatment, but biomass RGR and biomass per length were inversely correlated with native macrophyte biomass, which is consistent with competition. Our results unexpectedly showed water brownification to have more influence on lake invasion than climate warming at this temperature regime. Two pathways emerged for climate to interact with biological invasions in structuring novel communities: directly, if non-native species respond positively to climate change, and indirectly through species interactions, for instance, because water brownification impairs growth of native macrophytes and reduces biotic resistance to invasion.

A new and simple method for qualitative sampling of meiobenthos-communities

by Finn Viehberg

Viehberg, F.A., 2002. A new and simple method for qualitative sampling of meiobenthos-communities. Limnologica - Ecology and Management of Inland Waters 32, 350-351. DOI:10.1016/S0075-9511(02)80026-3

The introduced collecting method is based on a common entomologist exhauster, which is specified for the fieldwork in... more The introduced collecting method is based on a common entomologist exhauster, which is specified for the fieldwork in aquatic systems with a sieve-inset and a bottom-lid. The living
organisms are collected visually from the sampled debris and no further extraction is needed. The apparatus is designed for qualitative sampling of microcrustaceans (e.g. Cladocera, Copepoda, Ostracoda) and minimises disturbance at the sampling site.

Zooplankton interactions in an enclosure experiment: insights from stable isotope analyses

by Jonathan Grey

Freshwater Biology 2004

1. Density gradients of cladocerans and copepods were generated in an enclosure experiment to compare the impact on... more 1. Density gradients of cladocerans and copepods were generated in an enclosure experiment to compare the impact on the plankton of a filter feeder (Daphnia hyalina × galeata) with that of more selective feeders (calanoid and cyclopoid copepods). The experiment was conducted in situ over 25 days during spring in a mesotrophic lake, Schöhsee, Germany.

2. The plankton community was monitored regularly. Daphniids were able to graze on the phytoplankton present, which mainly consisted of small (<1000 μm3) species, whereas copepods did not show any impact on algae.

3. At the end of the experiment, Daphnia and remaining cyclopoid copepods were harvested and sorted manually, prior to analyses for stable isotopes of carbon and nitrogen. Daphniids from mesocosms stocked purely with differing densities of Daphnia showed little variability in stable isotope values, whereas those that thrived in enclosure bags together with copepods exhibited lower δ13C values.

4. The change in Daphniaδ13C indicates a change of food sources, modified by the presence of the copepods: the higher the mean abundance of copepods in the enclosures, the more 13C-depleted the daphniids. Increasing abundance of high nucleic acid (HNA) bacteria in the copepod bags may account for the trend in Daphniaδ13C via increased grazing on the bacteria themselves, or via grazing on phytoplankton utilising isotopically light CO2 from respiratory release.

5. Cyclopoid copepod stable isotope signatures were related to Daphnia and copepod abundances in copepod bags, suggesting that cyclopoids preyed on the available zooplankton.

Organic inclusions in lacustrine diatom frustules as a host for carbon and nitrogen isotopes.

by Jonathan Grey

Verh Int Verein Limnol 2006

Using stable isotope analyses to identify allochthonous inputs to Lake Naivasha mediated via the hippopotamus gut

by Jonathan Grey

Isotopes In Environmental & Health Studies 2002

The hippopotamus grazes nocturnally on land and resides in water during the day. Much of the ingested material must... more The hippopotamus grazes nocturnally on land and resides in water during the day. Much of the ingested material must therefore be defecated directly into the aquatic system and can thus be considered an allochthonous resource available to aquatic consumers. The utility of stable isotope analyses of carbon and nitrogen to distinguish hippo faecal matter from other potential basal resources was tested at Lake Naivasha, Kenya. Hippopotami proved faithful to a short grass diet although supplementary grazing of aquatic macrophytes was observed. The typical isotopic ratios of C4 grasses ingested were not altered substantially by gut processes, and were clearly distinct from algal and aquatic macrophyte isotopic ratios. However, marginal plants such as Cyperus papyrus exhibit C4 ratios, and so the technique is suitable only for use in localities where 'contamination' from such sources is negligible

Ferox Trout (Salmo trutta) as ‘Russian dolls’: complementary gut content and stable isotope analyses of the Loch Ness foodweb

by Jonathan Grey

Freshwater Biology 2002

1. Conventional collection methods for pelagic fish species (netting, trawling) are impractical or prohibited in Loch... more 1. Conventional collection methods for pelagic fish species (netting, trawling) are impractical or prohibited in Loch Ness, U.K. To investigate trophic relationships at the top
of the Loch Ness food web, an alternative strategy, angling, provided samples of the top predator, the purely piscivorous ferox trout (Salmo trutta).
2. The gut contents of these fish provided further samples of prey-fish, and subsequent examination of prey-fish guts revealed their dietary intake, analogous to the famous nested
‘Russian dolls’. Each trophic level separated by gut content analysis provided further complementary samples for stable isotope analysis and thus information on the longer
term, assimilated diet.
3. Ferox trout exhibited considerable cannibalism to supplement a diet of Arctic charr (Salvelinus alpinus). However, conspecifics stemmed from a lower isotopic baseline in relation to charr, so ferox trout exhibited a lower trophic level than predicted (4.3) by using the d15N values. Charr displayed dietary specialisation with increasing length, and isotopic values supported by the gut data placed the charr at a trophic level of 3.5. The isotope data
also indicated that charr carbon was primarily autochthonous in origin.

A revised model for lipid‐normalizing δ13C values from aquatic organisms, with implications for isotope mixing models

by Jonathan Grey

Journal of Applied Ecology 2006

1. Stable isotope analyses coupled with mixing models are being used increasingly to evaluate ecological management... more 1. Stable isotope analyses coupled with mixing models are being used increasingly to evaluate ecological management issues and questions. Such applications of stable isotope analyses often require simultaneous carbon and nitrogen analyses from the same sample. Correction of the carbon isotope values to take account of the varying content of 13C-depleted lipids is then frequently achieved by a lipid-normalization procedure using a model describing the relationship between change in δ13C following lipid removal and the original C:N ratio of a sample.
2. We evaluated the applicability of two widely used normalization models using empirical data for muscle tissue from a wide range of fish and for aquatic invertebrates. Neither normalization model proved satisfactory, and we present some modifications that greatly improve the fit of one of the models to the fish muscle data. For invertebrates we found no clear relationship between change in δ13C following lipid removal and the original C:N ratio.
3. We also examined the effect of lipid-normalization on the output of a mixing model designed to calculate the proportional contribution of prey items to the diet of a consumer. Mixing model output was greatly influenced by whether prey or consumer values alone or together were lipid-normalized and we urge caution in the interpretation of results from these models pending further experimental evidence.
4. Synthesis and applications. We describe a revised lipid-normalization model that should be applicable to a wide range of marine and freshwater fish species in studies applying stable isotope analyses to ecological management issues. However, we strongly adviseagainst applying these kinds of lipid-normalization models to aquatic invertebrate data..The interpretation of outputs from mixing models is greatly influenced by whether the
carbon isotope data have been lipid-normalized or not

Seasonal changes in the importance of the source of organic matter to the diet of zooplankton in Loch Ness, as indicated by stable isotope analysis

by Jonathan Grey

Limnology & Oceanography 2001

Seasonal variations in the stable isotope composition (d13C and d15N) of crustacean zooplankton and their putative... more Seasonal variations in the stable isotope composition (d13C and d15N) of crustacean zooplankton and their putative food sources in oligotrophic Loch Ness were recorded during 1998. Bulk particulate organic matter (POM) showed d13C values consistent with a terrestrial plant origin from the catchment and exhibited little seasonal variation, whereas POM d15N was more variable, probably due to associated microbial action. In contrast, phytoplankton d13C was relatively light and showed some seasonal variation, but d15N values were more constant. The isotopic signatures of both POM and phytoplankton remained sufficiently distinct from each other throughout the period of study to allow their relative contributions to zooplankton diet to be assessed. Zooplankton isotopic signatures shifted seasonally, reflecting a dietary switch from a reliance on allochthonous carbon derived from POM during winter and early spring to heavy dependence on algal production during summer. Annually, crustacean zooplankton in Loch Ness derive approximately 40% of their body carbon from allochthonous sources, likely mediated via microbial links. Separate determination of isotope ratios for the main zooplankton species allowed a more detailed trophic investigation. The most abundant zooplankton species in the loch, Eudiaptomus gracilis, incorporated appreciable allochthonous carbon even during the peak of phytoplankton productivity. By contrast, Daphnia hyalina grew mainly in late summer and autumn and derived almost 100% body carbon from algal sources. This study is the first to quantify such a seasonal switch in zooplankton dependence between allochthonous and autochthonous sources of
organic matter in a large lake.

Biogenic methane in freshwater food webs

by Jonathan Grey

Freshwater Biology 2011

1. It has long been known that substantial amounts of methane are produced in anoxic lake sediments, and the... more 1. It has long been known that substantial amounts of methane are produced in anoxic lake sediments, and the components of the methane cycle in lakes have been well described. At oxic–anoxic interfaces, methane-oxidising bacteria (MOB) convert methane to microbial biomass and can be highly productive. However, only recently has methane been recognised as a potentially important carbon and energy source for lake food webs, and some instances have also been reported of methane contribution to river food webs. Stable isotope analysis (SIA) has provided compelling evidence in this respect and has been supplemented by other lines of evidence.
2. In the benthic food webs of lakes, profundal chironomid larvae appear to be the main conduits for trophic transfer of biogenic methane via grazing on MOB. The mode of feeding of these larvae and the microhabitats they generate both promote larval ability to exploit MOB production. Support to chironomid larvae from methane is rather widespread, but its degree is highly variable; estimates suggest that in some lakes methane-carbon might contribute more than 60% of chironomid carbon biomass.
3. Evidence of crustacean zooplankton in lakes deriving part of their carbon from methane is currently more limited. Reports from some lakes have indicated Daphnia with a substantial (>50%) contribution of methane-carbon in their biomass. However, for this to happen, an oxic–anoxic interface where sufficient MOB production can occur needs to be within the range of vertical migrations by zooplankton, which may only rarely be the case. Hence, a significant methane subsidy of pelagic food webs in lakes is probably much less widespread than for benthic food webs.
4. There is also recent and currently very limited evidence that some stream benthos derives biomass carbon (reported values up to 30%) from methane. This can occur in stagnant backwater pools where conditions can be analogous to those in lake sediments. However, groundwater aquifers can also supply water supersaturated with methane to some rivers, providing a basis for a microbially-mediated transfer of methane-carbon to river benthos.
5. Evidence for significant transfer of methane-derived carbon to higher trophic levels is still very limited. Within some lakes, those fish species that feed extensively on chironomid larvae can derive a substantial part (perhaps up to 20%) of their carbon biomass from methane. It is also likely that methane-carbon produced in lakes or rivers is exported to riparian ecosystems when emerging chironomids or other insects are eaten by invertebrate or avian predators.
6. We argue that conceptual models of freshwater food webs, and especially those for lakes, need to be modified to enable incorporation of biogenic methane as a carbon and energy source. For some types of lakes, carbon and energy budgets certainly need to take account of the production and utilisation of biogenic methane, and the accumulating evidence indicates that this is a more widespread phenomenon that has generally been acknowledged hitherto

Isotopic variation complicates analysis of trophic relations within the fish community of Plußsee: a small, deep, stratifying lake

by Jonathan Grey

Archiv Für Hydrobiologie 2006

Analysis of carbon and nitrogen stable isotopes has allowed freshwater ecologists to examine lake food webs in... more Analysis of carbon and nitrogen stable isotopes has allowed freshwater ecologists to examine lake food webs in increasing detail. Many such studies have highlighted the existence of separate within-lake pelagic and benthic-littoral food webs but are typically conducted on large (>10 km2) lakes, whereas the majority of lakes are actually relatively small. We used stable isotope analysis (δ13C & δ15N) to examine trophic interactions between fish and their prey in Plußsee, as an example of a small, stratifying lake, and to determine whether separate pelagic/benthic-littoral food webs could be distinguished in such systems. Our results indicate that the Plußsee food web was complicated, and due to extensive intra-annual isotopic variation in zooplankton (e. g. cladoceran δ13C annual range = 25.6 ‰), it may be impossible to definitively assign consumers from small, eutrophic stratified lakes to pelagic or benthic-littoral food webs. We present evidence that some components of the Plußsee food web (large bream) may be subsidised by carbon of methanogenic origin.

Stable isotope analysis of zooplankton carbon nutrition in humic lakes

by Jonathan Grey

Oikos 1999

Carbon stable isotopic composition was determined for zooplankton, POM, DOM and, in two cases, phytoplankton from 12... more Carbon stable isotopic composition was determined for zooplankton, POM, DOM and, in two cases, phytoplankton from 12 small forest lakes in southern Finland during summer 1997. The lakes were selected to provide a wide range of water colour (12-365 mg Pt l(-1)) but minimum variability in other limnological characteristics. POM and DOM showed similar stable isotope ratios (delta(13)C around - 28 parts per thousand, comparable with values commonly reported for terrestrial C-3 vegetation) and these showed no relationship to water colour. Zooplankton were consistently depleted in C-13 relative to the bulk POM on which they might feed. This relative C-13 depletion increased significantly with water colour. Therefore the zooplankton must have been feeding selectively on isotopically light food sources and doing so to a greater extent in the more humic lakes. Although phytoplankton were C-13-depleted relative to POM, their abundance and their variability between lakes did not appear to explain adequately the observed trend in zooplankton stable isotope ratios. We propose that some zooplankton were grazing on methanotrophic bacteria which utilise isotopically light methane derived from allochthonous organic matter decomposed under anaerobic conditions. This process would be more pronounced in coloured lakes with a higher loading of allochthonous organic matter and greater development of hypolimnetic anoxia.

Lipid extraction has little effect on the δ15N of aquatic consumers

by Jonathan Grey

Limnology & Oceanography Methods 2007

Proper application of stable isotopes (e.g., δ15N and δ13C) to food web analysis requires an understanding of all... more Proper application of stable isotopes (e.g., δ15N and δ13C) to food web analysis requires an understanding of all nondietary factors that contribute to isotopic variability. Lipid extraction is often used during stable isotope analysis (SIA), because synthesized lipids have a low δ13C and can mask the δ13C of a consumer’s diet. Recent studies indicate that lipid extraction intended to adjust δ13C may also cause shifts in δ15N, but the magnitude of and reasons for the shift are highly uncertain. We examined a large data set (n = 854) for effects of lipid extraction (using Bligh and Dyer’s [1959] chloroform-methanol solvent mixtures) on the δ15N of aquatic consumers. We found no effect of chemically extracting lipids on the δ15N of whole zooplankton, unionid mussels, and fish liver samples, and found a small increase in fish muscle δ15N of ~0.4‰. We also detected a negative relationship between the shift in δ15N following extraction and the C:N ratio in muscle tissue, suggesting that effects of extraction were greater for tissue with lower lipid content. As long as appropriate techniques such as those from Bligh and Dyer (1959) are used, effects of lipid extraction on δ15N of aquatic consumers need not be a major consideration in the SIA of food webs.

Stable isotope analyses provide new insights into ecological plasticity in a mixohaline population of European eel

by Jonathan Grey

Oecologia 2005

Recent studies have shown that anguillid eel populations in habitats spanning the marine–freshwater ecotone can... more Recent studies have shown that anguillid eel populations in habitats spanning the marine–freshwater ecotone can display extreme plasticity in the range of catadromy expressed by individual fishes. Carbon and nitrogen stable isotope analysis was used to differentiate between European eels (Anguilla anguilla) collected along a short (2 km) salinity gradient ranging from <1‰ to ~30‰ in Lough Ahalia, a tidal Atlantic lake system. Significant differences were recorded in mean d13C, d15N and C:N values from eels collected from fresh, brackish and marine-dominated basins. A discriminant analysis using these predictor variables correctly classified ca. 85% of eels to salinity zone, allowing eels to be classified as freshwater (FW), brackish (BW) or marine (MW) residents. The results of the discriminant analysis also suggested that a significant proportion of eels moved between habitats (especially between FW and BW). Comparisons of several key population parameters showed significant variation between eels resident in different salinity zones. Mean condition and estimated age was significantly lower in MW eels, whilst observed length at age (a correlate of growth) was significantly higher in MW eels, intermediate in BW and lowest in FW eels. This study has demonstrated that the ecology of eels found along a short salinity gradient can be extremely plastic and that stable isotope analysis has considerable utility in demonstrating intra-population variation in diadromous fishes.