Changes In Trophic Status Research Articles

CO2 Saturation and Trophic Shift Induced by Microbial Metabolic Processes in a River-Dominated Ocean Margin (Tropical Shallow Lagoon, Chilika, India)

ABSTRACTAn effort has been made for the first time in Asia's largest brackish water lagoon, Chilika, to investigate the spatio-temporal variability in primary productivity (PP), bacterial productivity (BP), bacterial abundance (BA), bacterial respiration (BR) and bacterial growth efficiency (BGE) in relation to partial pressure of CO2 (pCO2) and CO2 air–water flux and the resultant trophic switchover. Annually, PP ranged between 24 and 376 µg C L−1 d−1 with significantly low values throughout the monsoon (MN), caused by light limitation due to inputs of riverine suspended matter. On the contrary, BP and BR ranged from 11.5 to 186.3 µg C L−1 d−1 and from 14.1 to 389.4 µg C L−1 d−1, respectively, with exceptionally higher values during MN. A wide spatial and temporal variation in the lagoon trophic status was apparent from BP/PP (0.05–6.4) and PP/BR (0.10–18.2) ratios. The seasonal shift in net pelagic production from autotrophy to heterotrophy due to terrestrial organic matter inputs via rivers, enhanced the bacterial metabolism during the MN, as evident from the high pCO2 (10,134 µatm) and CO2 air–water flux (714 mm m−2 d−1). Large variability in BGE and BP/PP ratios especially during MN led to high bacteria-mediated carbon fluxes which was evident from significantly high bacterial carbon demand (BCD >100% of PP) during this season. This suggested that the net amount of organic carbon (either dissolved or particulate form) synthesized by primary producers in the lagoon was not sufficient to satisfy the bacterial carbon requirements. Lagoon sustained low to moderate autotrophic–heterotrophic coupling with annual mean BCD of 231% relative to the primary production, which depicted that bacterioplankton are the mainstay of the lagoon biogeochemical cycles and principal players that bring changes in trophic status. Study disclosed that the high CO2 supersaturation and oxygen undersaturation during MN was attributed to the increased heterotrophic respiration (in excess of PP) fuelled by allochthonous organic matter. On a spatial scale, lagoon sectors such as south sector, central sector and outer channel recorded “net autotrophic,” while the northern sector showed “net heterotrophic” throughout the study period.

Changes in water quality and trophic status associated with cage aquaculture in Lake Maninjau, Indonesia

The cage aquaculture unquestionably has been degrading lake water quality by increasing nutrients and organic carbon in lake water and sediments. The question is to what extend this condition affects other key indictors such as the temporal changes in trophic status and the thickness of anoxic hypolimnion layer where the anoxic water column is moving upward pushing up the oxic epilimnion layer. The condition in Lake Maninjau could be worse since the lake is steadily producing sulfide which can cause not only oxygen depletion in the water column but also the phosphate release from the sediments. The study is based on the long term monitoring data from on going research for about 8 years observation. The results indeed show the anoxic water column is moving upward increasing the thickness of anoxic hypolimnion layer and decreasing epilimnion layer from 30 m to 10 m depth. The trophic status of the lake also has changed from mesotrophic to eutrophic decreasing the water transparency to even a critical level < 1m. The months of July to September with prolonged hot season could be the critical time for trophic condition for the lake. The results suggest that determination of these conditions further could help identify and predict the critical time for possibility of fish kill.

Recent and historic eutrophication of an island lake in northern Lake Michigan, USA

Lake Geneserath is located on Beaver Island in northern Lake Michigan, USA. It exhibits evidence of recent eutrophication, despite limited anthropogenic activity in the watershed. The primary goal of this study was to determine the time scale of trophic state change in Lake Geneserath and the influence of multiple stressors. Sediment cores from the shallow northern and deep southern basins of Lake Geneserath were collected and sectioned at 0.5-cm intervals over the topmost 10 cm and at 1.0-cm intervals for the remainder of the core. Total phosphorus, percent organic matter, and chlorophyll derivative concentrations were measured in each section, and core chronology was established using 210Pb. Diatom assemblages were quantified from evenly distributed sections in each core. Geochemical variables in the southern basin core indicate historic (AD ~ 1450–1650) and modern (1848-present) periods of eutrophication, coincident with human disturbance and climate warming events. Geochemical variables in the northern basin core confirm accelerating eutrophication after 1950. The fossil diatom assemblages in both basins shifted from species favoring low nutrients to more nutrient-tolerant taxa. A change from periphytic to planktonic diatom species, concurrent with reduced percent organic matter in the northern basin core, indicates rising water level during the second half of the twentieth century. Lake Geneserath was subject to multiple stressors and its trophic status increased with recent residential development and climate warming.

Determination of Trophic State Changes with Diel Dissolved Oxygen: A Case Study in a Shallow Lake.

Current trophic state indices (TSI) have been reported to have limitations in assessing changes in eutrophication status of shallow waters. This study aimed to use intensive measurements on dissolved oxygen (DO) to improve the determination of tropic state changes. The authors deployed an environment monitoring buoy in a eutrophic shallow lake and recorded water temperature, DO, and chlorophyll-a concentrations at 15-minute intervals for two 1-year periods: from August 2008 to July 2009 and from August 2013 to July 2014. In addition, they recorded water levels over the same periods and collected water samples for nutrient analysis. The authors analyzed the high-time resolution DO records, compared the diel DO trends between the two 1-year periods, and proposed a new TSI using DO. They found that analyzing the change in diel DO ranges can improve commonly used methods for classifying trophic states and assessing the change of eutrophication status of waterbodies.

Diatom response to Asian monsoon variability during the Holocene in a deep lake at the southeastern margin of the Tibetan Plateau

Diatom analysis was applied to a 5.56‐m‐long sediment core, spanning the last 7.8 ka, from Lake Chenghai on the southeastern margin of the Tibetan Plateau. Diatom assemblages are dominated by Cyclotella rhomboideo‐elliptica, Cyclostephanos dubius and small fragilarioid and periphytic taxa. These diatom taxa are interpreted to be sensitive to changes in trophic status and/or turbulence of the water column that are probably related to variations in precipitation and temperature induced through the Asian monsoon. High abundances of C. dubius, Staurosirella pinnata and periphytic taxa suggest higher trophic status (mesotrophic) and stronger turbulence in the water column in the middle Holocene (7.8–4.5 ka BP). During the period of 4.5–2.3 ka BP, a decline in C. dubius, S. pinnata and periphytic taxa, and an increase in C. rhomboideo‐elliptica with the appearance of Cyclotella ocellata suggests a response to decreasing trophic level (more oligotrophic). After 2.3 ka BP, the persistent increases in C. rhomboideo‐elliptica and the loss of Cyclotella ocellata mirrored a further decrease in trophic level (oligotrophic). The main changes in the diatom assemblages in the Lake Chenghai sediment core reflect direct and indirect climate forcing and in particular, the strength of monsoonal precipitation, which in turn corresponds to orbitally induced variability in Northern Hemisphere summer solar insolation since 7.8 ka BP.

Patterns in benthic biodiversity link lake trophic status to structure and potential function of three large, deep lakes.

Relative to their scarcity, large, deep lakes support a large proportion of the world’s freshwater species. This biodiversity is threatened by human development and is in need of conservation. Direct comparison of biodiversity is the basis of biological monitoring for conservation but is difficult to conduct between large, insular ecosystems. The objective of our study was to conduct such a comparison of benthic biodiversity between three of the world’s largest lakes: Lake Tahoe, USA; Lake Hövsgöl, Mongolia; and Crater Lake, USA. We examined biodiversity of common benthic organism, the non-biting midges (Chironomidae) and determined lake trophic status using chironomid-based lake typology, tested whether community structure was similar between the three lakes despite geographic distance; and tested whether chironomid diversity would show significant variation within and between lakes. Typology analysis indicated that Lake Hövsgöl was ultra-oligotrophic, Crater Lake was oligotrophic, and Lake Tahoe was borderline oligotrophic/mesotrophic. These results were similar to traditional pelagic measures of lake trophic status for Lake Hövsgöl and Crater Lake but differed for Lake Tahoe, which has been designated as ultra-oligotrophic by traditional pelagic measures such as transparency found in the literature. Analysis of similarity showed that Lake Tahoe and Lake Hövsgöl chironomid communities were more similar to each other than either was to Crater Lake communities. Diversity varied between the three lakes and spatially within each lake. This research shows that chironomid communities from these large lakes were sensitive to trophic conditions. Chironomid communities were similar between the deep environments of Lake Hövsgöl and Lake Tahoe, indicating that chironomid communities from these lakes may be useful in comparing trophic state changes in large lakes. Spatial variation in Lake Tahoe’s diversity is indicative of differential response of chironomid communities to nutrient enrichment which may be an indication of changes in trophic state within and across habitats.

Recent changes in production in oligotrophic Uinta Mountain lakes, Utah, identified using paleolimnology

We use multiple proxies from lake sediment records of six remote alpine lakes in the Uinta Mountains, Utah, to investigate primary production and potential drivers of changes in trophic status over the last two centuries. Chlorophyll a, chlorophyll a flux, and percentage of organic matter (determined by loss on ignition) increase beginning in the mid‐20th century in five of the six alpine study lakes, indicating increasing production. These changes tend to coincide with decreasing sedimentary δ15N and increasing abundance of the nitrophilous diatom Asterionella formosa. An earlier, more subtle change in diatom community composition and δ15N indicates that human activities prior to 1950 had measurable effects on these lake ecosystems, although no corresponding increase in primary production was observed at this time. Multiple factors can lead to increased primary production at alpine sites, but the evidence presented shows that enhanced atmospheric deposition of nitrogen and phosphorus explain the recent, more prominent increase in production. Although previous research has suggested that alpine lakes are nitrogen limited, our results suggest nutrient limitation varies spatially and temporally, and is complex in these oligotrophic systems. This and other factors, such as catchment characteristics, will affect the sensitivity of a lake to atmospheric deposition–fertilizing effects. The changes observed show that remote lakes are vulnerable to long‐distance transport of nutrients, and that the risk of eutrophication could be intensified by increased nutrient inputs from expanding mining, fossil fuel combustion, and agriculture and by rapid warming predicted for the southwest.

Direct and Indirect Effects of Fish on Invertebrates and Tiger Salamanders in Prairie Pothole Wetlands

Hydrologic changes have facilitated increased fish abundance in wetlands of the North American prairie pothole region (PPR). Declines in PPR wetland condition have been attributed to fish-generated changes in trophic state, and reduced abundance and diversity of plants, invertebrates, and non-fish vertebrates. We sought to gain insight into mechanisms for fish impacts on wetland condition by evaluating their direct and indirect effects on invertebrates and salamanders. We quantified relationships among trophic state variables (chlorophyll a, nitrogen, phosphorus, total suspended solids, turbidity); fish and larval tiger salamander biomass; plant cover and taxon richness; and invertebrate density, biomass, and taxon richness in 34 continuously-flooded wetlands in Iowa. Path analysis results supported hypotheses that fish indirectly caused declines in invertebrate taxon richness by increasing turbidity, which in turn reduced plant cover. Path analysis results also supported the hypothesis that fish directly reduced salamander biomass by predation. Fish biomass increased with increasing water depth. Therefore, deeper wetlands were generally in poorer condition than relatively shallow ecosystems. Our findings support conclusions of previous investigators that regardless of other management strategies, actions limiting fish biomass in PPR wetlands (e.g., eliminating surface-water connections to other aquatic systems) are essential to restore health and functions of these ecosystems.

Genetic diversity of upper Lake Constance whitefish Coregonus spp. under the influence of fisheries: a DNA study based on archived scale samples from 1932, 1975 and 2006.

This investigation examined changes in the genetic diversity of pelagic upper Lake Constance (ULC) whitefish Coregonus wartmanni population before and after the alteration of fishery methods and management from 1932 to 2006. The study spans a period of pronounced changes in trophic status of the lake and transitions from traditional relatively unselective pelagic seine (Klusgarn) fishing to highly size-selective nylon gillnet techniques. In addition, supportive breeding and stocking became most popular during the phase of eutrophication in the 1970s. The main hypothesis is that size-selective fisheries and breeding lead to an overall decrease in genetic variability over time. A total of 215 archived C. wartmanni scale samples from 1932, 1975 and 2006 were analysed by genotyping 11 microsatellite loci. A comparison of population genetic parameters, including allelic richness, observed and expected heterozygosities, and estimates of effective population sizes, suggests that the genetic diversity of C. wartmanni population has not decreased. The appearance of new alleles in the gene pool in 1975 and 2006 may be indicative of admixture with other forms in the lake or with stocked allochthonous forms. Overall, the fisheries management practice in ULC, including the effects of size-selective fisheries, supportive breeding and stocking, have not significantly altered the genetic diversity of Coregonus spp. over an 80 year period.

Mid-late Holocene lake levels and trophic states of a shallow lake from the southern Pampa plain, Argentina

Changes in trophic status of the shallow Lake La Brava (southeastern Pampa plain of Argentina) are evaluated based on geochemical, biological and sedimentological data. Based on a conceptual framework, we propose that the lake level defines the water column mixing conditions, affecting internal lake processes and determining the transparency of lake water. The shift between alternative states is reconstructed for the last ~4800 years. Four main lake stages have been recognized, although short term shifts characterize all stages. A turbid phytoplankton-dominated state prevailed between ~4700-4500 cal yr before present (BP). Drier conditions and low lake levels pushed the lake to a clear state until 2000 cal yr BP. Afterwards the lake switched back to a turbid state, and these conditions persisted until ~200 cal yr BP. In the last 200 years, the lake switched back and forth between clear and turbid states. The latter represents the modern conditions of the lake since ~1950 AD. These shifts can be attributed mainly to climatic drivers.

Long-term variations in primary production in a eutrophic sub-estuary. II. Interannual variations and modeling

A 19 yr time series of annual primary production in the eutrophic Rhode River sub- estuary of the Chesapeake Bay in Maryland (USA) was analyzed in relation to climatological and ecological factors. The objectives of this work were to understand factors controlling interannual variations in primary production in a eutrophic estuary and develop a model for tracking future variations in production, in the absence of direct measurements of photosynthetic carbon uptake. Annual production (PA) averaged 328 (range 152 to 612) g C m −2 yr −1 . Interannual variability was statistically significant, but there was no significant linear trend or significant non-random varia- tions over the available 19 yr. Climatological indices based on North Atlantic Oscillation or flow of the Susquehanna River, the principal N source to the upper Chesapeake Bay, were not significant predictors of PA. A classification of years based on magnitude of the spring dinoflagellate bloom and timing of nitrate depletion was a significant predictor of PA. Phytoplankton biomass, B, and the light saturated photosynthetic rate normalized to chlorophyll, P B, were of similar magnitude in their influence on the variance in PA. The high degree of variability in P B max weakened efforts to model both daily and annual production from measurements of chlorophyll and light attenuation. Between 4 and 15 yr of measurements of chlorophyll and light attenuation would be needed to detect a change in trophic status of the sub-estuary, depending on the level of reduction achieved in PA. Average daily production would have to be reduced below 1052 mg C m −2 d −1 to achieve mesotrophic status.

The need for ecological monitoring of freshwaters in a changing world: a case study of Lakes Annecy, Bourget, and Geneva

Lakes Annecy, Bourget, and Geneva are large, deep carbonated peri-alpine lakes in eastern France. They are located in the same ecoregion but have been subject to differing degrees of anthropogenic pressure over the past decades. A comparative analysis of these ecosystems can therefore provide valuable information on how the lakes have responded to changes in phosphorus runoff, fish management practices, and global warming. Each of these lakes has undergone a restoration process, and changes in water quality and trophic state, as measured using parameters like transparency, chlorophyll a, nutrient concentrations, and phytoplankton biomass and structure, can be used to evaluate efforts made to preserve these ecosystems. Our results reveal that (1) peri-alpine lakes are exemplary cases of restoration in the world where freshwater eutrophication is on the increase, and (2) efforts must be maintained because of the new context of climate change, the effects of which on the quality and the ecological functioning of lakes are still poorly understood.

Long‐term changes in hypoxia and soluble reactive phosphorus in the hypolimnion of a large temperate lake: consequences of a climate regime shift

The (Lower) Lake of Zurich provides an ideal system for studying the long-term impact of environmental change on deep-water hypoxia because of its sensitivity to climatic forcing, its history of eutrophication and subsequent oligotrophication, and the quality and length of its data set. Based on 39years (1972-2010) of measured profiles of temperature, oxygen concentration and phosphorus (P) concentration, the potentially confounding effects of oligotrophication and climatic forcing on the occurrence and extent of deep-water hypoxia in the lake were investigated. The time-series of Nürnberg's hypoxic factor (HF) for the lake can be divided into three distinct segments: (i) a segment of consistently low HF from 1972 to the late-1980s climate regime shift (CRS); (ii) a transitional segment between the late-1980s CRS and approximately 2000 within which the HF was highly variable; and (iii) a segment of consistently high HF thereafter. The increase in hypoxia during the study period was not a consequence of a change in trophic status, as the lake underwent oligotrophication as a result of reduced external P loading during this time. Instead, wavelet analysis suggests that changes in the lake's mixing regime, initiated by the late-1980s CRS, ultimately led to a delayed but abrupt decrease in the deep-water oxygen concentration, resulting in a general expansion of the hypoxic zone in autumn. Even after detrending to remove long-term effects, the concentration of soluble reactive P in the bottom water of the lake was highly correlated with various measures of hypoxia, providing quantitative evidence supporting the probable effect of hypoxia on internal P loading. Such climate-induced, ecosystem-scale changes, which may result in undesirable effects such as a decline in water quality and a reduction in coldwater fish habitats, provide further evidence for the vulnerability of large temperate lakes to predicted increases in global air temperature.

Phytoplankton absorption and the chlorophyll a-specific absorption coefficient in dynamic Onondaga Lake

Phytoplankton absorption and its dependence on the concentration of chlorophyll a(Chl-a), as represented by the Chl-a–specific absorption coefficient (a*φ(λ)), is important to support models of growth and for bio-optical remote sensing algorithms to retrieve Chl-a. The dynamics of the phytoplankton absorption coefficient (aφ(λ)) and a*φ(λ), and their dependencies on Chl-a, are described for Onondaga Lake, New York, over a 6-year period for which major changes in trophic state, Chl-a, and community composition occurred. Strong positive dependencies of aφ(λ) on Chl-aare reported for absorption peaks in both the blue and red spectral regions that are qualitatively similar to relationships for ocean waters but differ quantitatively. Average values of a*φ at wavelengths of 440 and 676 nm were 0.0347 and 0.0171 m2 mg−1, respectively, with coefficients of variation of 37 and 31%. Significant negative relationships between a*φ and Chl-a were observed for blue and green wavelengths that were qualitatively consistent with the influences of pigment packaging and the contribution of accessory pigments to absorption. The operation of these influences is demonstrated through various forms of data analysis that resolved the following significant relationships: (1) negative dependence of the ratio aφ(440):aφ(676) on Chl-a; (2) flattening of aφ spectra in the blue and increases at the red maximum, with increases in Chl-a; and (3) negative dependence of aφ(490):aφ(676) on Chl-a. Values of a*φ(440) and a*φ(676) obtained for Onondaga Lake are considered in the context of the limited population reported for other inland waters and selected marine systems.

A null model to study community structure of microcrustacean assemblages in north Patagonian urban wetlands (39-41°S, Chile)

The inland water bodies of northern Chilean Patagonia (38-41°S) have many lakes, wetlands and ponds with different littoral and zooplanktonic crustacean assemblages. This study presents field observations of species associations sampled from shallow wetlands located in the urban zones of Valdivia (39°S) and Puerto Montt (41°S). A species presence-absence matrix was created for calculating the Jaccard Index of community similarity and for testing null models of species associations, with the aim of determining whether species associations are random or not. The results of the Jaccard Index calculations indicated the existence of non-defined groups. The results of the null model analysis denoted the presence of regulating factors for Valdivia wetlands, whereas for Puerto Montt wetlands no such factors could be demonstrated. The outcomes of both the Jaccard Index and the significant null model analysis agree with previous ecological descriptions of changes in trophic status as a consequence of changes in the surrounding basin as a determinant of species associations. The ecology of these communities is also discussed.

A sediment record of trophic state change in an Arkansas (USA) reservoir

Reservoir sediments are used cautiously in paleolimnological studies because of dating uncertain- ties, possible sediment disturbances and even concerns that indicators of trophic status may behave differently in reservoirs as opposed to natural lakes. We measured loss on ignition (LOI), carbon to nitrogen ratio (C:N), diatom abundance, total nitrogen (TN), total phosphorus (TP), TN:TP ratio, and carbon and nitrogen isotopes (d 13 Ca ndd 15 N) in an 83-cm sediment core to track

Assessing the responses of aquatic macrophytes to the application of a lanthanum modified bentonite clay, at Loch Flemington, Scotland, UK

Loch Flemington is a shallow lake of international conservation and scientific importance. In recent decades, its status has declined as a result of eutrophication and the establishment of non-native invasive aquatic macrophytes. As previous research had identified the lake bed sediments as an important source of phosphorus (P), the P-capping material Phoslock® was applied to improve water quality. This article documents the responses of the aquatic macrophyte community by comparing data collected between 1988 and 2011. Summer water-column total P concentrations decreased significantly and water clarity increased following treatment. Aquatic plant colonisation depth increased and plant coverage of the lake bed extended. However, the submerged vegetation remained dominated by the non-native Elodea canadensis Michx. Aquatic macrophyte community metrics indicated no significant change in trophic status. Species richness and the number of ‘natural’ eutrophic characteristic species remained broadly similar with no records of rare species of conservation interest. Loch Flemington is still classified as being in ‘unfavourable no change’ condition based on its aquatic macrophytes despite the water quality improvements. The implications of these results are discussed in relation to the future management of Loch Flemington and in the wider context of trying to improve our understanding of lake restoration processes.

The eutrophication history of a tropical water supply reservoir in Brazil

Guarapiranga Reservoir is the second most important public water supply in Sao Paulo, Brazil and has been eutrophic for several decades. We inferred the major ecological shifts for the period 1919–2010 related to multiple stressors (forest flooding, hydrological change, use of algicide and eutrophication), using geochemistry (TOC, TN, TP, C/N, δ15N, δ13C) and diatom assemblages in a short (75-cm) sediment core. Thirty-two diatom species were abundant in the core and stratigraphically constrained incremental sum of squares analysis enabled identification of three diatom zones and four subzones, i.e. depths at which marked changes in species composition occurred. Early diatom assemblages were dominated by benthic, oligotrophic taxa, mainly Eunotia, influenced by flooded vegetation after dam construction. A shift to dominance by a planktonic species (Eunotia tukanorum) occurred ca. 1932, during the period of initial physical disturbance and early use of the water body as a public water supply. Diatoms and geochemical variables show that the reservoir was oligotrophic from ~1919 to 1947. Eutrophication began ~1975 and by the early 1980s the reservoir had become eutrophic, in response to an explosive increase in human population in the watershed. Severe cultural eutrophication has persisted since ~1990. Higher concentrations of copper in the sediments, beginning in 1991, reflect the increased use of copper sulfate to control cyanobacteria blooms and provide a chronological marker. Higher δ15N values in recent sediments indicate greater sewage inputs and low C/N values reflect the predominant contribution of algae to sediment organic matter. Eutrophic taxa Cyclotella meneghiniana and Nitzschia sp. dominate recent diatom assemblages, along with Aulacoseira granulata, a species that is tolerant of copper sulfate. Diatom assemblages reflect multiple stressors, however, geochemical information provides a better understanding of the early phase of the reservoir. Paleolimnologically documented trophic state changes in this important drinking water supply are largely attributable to increased urbanization of the drainage basin and inputs of sewage. Management efforts should focus on mitigating this nutrient source.

Midwater zooplankton and suspended particle dynamics in the North Pacific Subtropical Gyre: A stable isotope perspective

We used amino acid (AA) compound‐specific isotope analysis (δ15NAA and δ13CAA values) of midwater zooplankton and suspended particles to examine their dynamics in the mesopelagic zone. Suspended particle δ15NAA values increased by up to 14‰ with depth, whereas particle trophic status (measured as trophic position, TP) remained constant at 1.6 ± 0.07. Applying a Rayleigh distillation model to these results gave an observed kinetic isotope fractionation of 5.7 ± 0.4‰, similar to that previously measured for protein hydrolysis. AA‐based degradation index values also decreased with depth on the particles, whereas a measure of heterotrophic resynthesis (ΣV) remained constant at 1.2 ± 0.3. The main mechanism driving 15N enrichment of suspended particles appears to be isotope fractionation associated with heterotrophic degradation, rather than a change in trophic status or N source with depth. In zooplankton the “source” AA phenylalanine (Phe) became 15N enriched by up to 3.5‰ with depth, whereas zooplankton TP increased by up to 0.65 between the surface ocean and midwaters. Both changes in the δ15N values of food resources at the base of the zooplankton food web and changes in zooplankton TP drive observed zooplankton 15N enrichment with depth. Midwater zooplankton δ15NPhe values were lower by 5–8‰ compared with suspended particles, indicating this organic matter pool is not a significant zooplankton food resource at depth. Instead, 62–88% of the N sustaining midwater zooplankton is surface derived, obtained through consumption of sinking particles, carnivory of vertical migrants, or direct feeding in surface waters at night.

Bioindicators of climate and trophic state in lowland and highland aquatic ecosystems of the Northern Neotropics

Chironomids, diatoms and microcrustaceans that inhabit aquatic ecosystems of the Northern Neotropics are abundant and diverse. Some species are highly sensitive to changes in water chemical composition and trophic state. This study was undertaken as a first step in developing transfer functions to infer past environmental conditions in the Northern lowland Neotropics. Bioindicator species abundances were related to multiple environmental variables to exploit their use as environmental and paleoenvironmental indicators. We collected and analyzed water and surface sediment samples from 63 waterbodies located along a broad trophic state gradient and steep gradients of altitude (approximately 0-1 560 m.a.s.l.) and precipitation (approximately 400-3200 mm/y), from NW Yucatán Peninsula (Mexico) to southern Guatemala. We related 14 limnological variables to relative abundances of 282 diatom species, 66 chironomid morphospecies, 51 species of cladocerans, 29 non-marine ostracode species and six freshwater calanoid copepods. Multivariate statistics indicated that bicarbonate is the strongest driver of chironomid and copepod distribution. Trophic state is the second most important factor that determines chironomid distribution. Conductivity, which is related to the precipitation gradient and marine influence on the Yucatán Peninsula, is the main variable that shapes diatom, ostracode and cladoceran communities. Diatoms, chironomids and cladocerans displayed higher diversities (H = 2.4-2.6) than ostracodes and copepods (H = 0.7-1.8). Species richness and diversity were greater at lower elevations (< 450 m.a.s.l.) than at higher elevations in Guatemala. Distribution and diversity of bioindicators are influenced by multiple factors including altitude, precipitation, water chemistry, trophic state and human impact.