Pelagic Regions Research Articles

Long-term trends in seasonal plankton dynamics in Lake Mead (Nevada-Arizona, USA) and implications for climate change

Over the past few decades, Lake Mead (Nevada-Arizona, USA) has experienced multiple ecosystem stressors including drought, increased water demand, and establishment of invasive species (quagga mussels, gizzard shad). Despite these potential stressors, zooplankton and phytoplankton community dynamics in the pelagic regions of Lake Mead have generally followed consistent seasonal patterns. Long-term monitoring results (2007–2015) show that zooplankton and phytoplankton communities remain relatively stable in Lake Mead on an inter-annual basis, but are susceptible to shifts caused by extreme climate fluctuations and alterations in mixing regimes. A warm winter characterized by low snowpack in 2014/2015 preceded a large bloom of toxic cyanobacteria (Microcystis aeruginosa Kutzing) in Las Vegas Bay the following summer. Large bloom events are rare in Lake Mead; however, under future climate scenarios, these types of events may become more frequent. Because of the consistency of plankton community dynamics over an extended period of time, Lake Mead offers an ideal system for the study of future climate change impacts. This study aims to characterize the response of plankton communities in Lake Mead to both linear and dynamic environmental changes.

Copepod assemblage structure (Crustacea: Copepoda) along a longitudinal environmental gradient in a tropical river-floodplain system, Brazil

Abstract Aim: In this study, we analyzed the structure of the copepod community along a longitudinal axis of the last undammed stretch of the Upper Paraná River floodplain, downstream of Porto Primavera reservoir. We hypothesized that (i) copepod assemblage would show an increase in the abundance of organisms, species richness and specific diversity with the distance from the Porto Primavera reservoir, due to the arrival of species from adjacent lakes and tributaries; (ii) copepod abundance and species richness would be higher in tributaries and adjacent lakes located at the floodplain, which show higher environmental heterogeneity and function as a species source; and (iii) spatial patterns of copepod assemblage structure are related to the environmental gradient, since copepods show a rapid response to the variation of environmental conditions. Method Sampling was performed quarterly from August 2013 to July 2014, at the subsurface of the pelagic region. Sampling sites were located along the Paraná River, in seven of its tributaries, and in eight lakes located in its floodplain. Results We identified 29 species belonging to two families: Cyclopidae (21 species) and Diaptomidae (eight species). On contrary of our first hypothesis, ANOVA results showed a significant decrease in copepod abundance in the downstream direction. Similarly, the tributaries showed higher values of ecological attributes in the upstream stretch. RDA results showed that in axis 1, the Paraná River and the lakes were positively correlated with conductivity, pH, temperature and suspended organic matter, characterized by the most abundant copepod species. The tributaries, on the other hand, were negatively correlated with turbidity, total nitrogen, total phosphorus and depth. Axis 2 showed a positive correlation of the Paraná River with conductivity and suspended organic matter, and a higher abundance of Argyrodiaptomus azevedoi, Notodiaptomus iheringi, N. henseni, N. cearensis and N. cf. spinuliferus. On the other hand, the lakes were positively correlated with temperature and pH and also with a higher abundance of Thermocyclops minutus and T. decipiens. Conclusion We highlight the importance of the maintenance of the ecological integrity of the Upper Paraná River floodplain, especially of its tributaries, in order to support the high biodiversity registered in the last undammed stretch. Furthermore, we highlight the importance of manipulation of water levels in reservoirs located upstream of this stretch, a necessary condition to maintain the hydrological connectivity between floodplain habitats and the main river.

Zooplankton abundance in the pelagic region of Lake Kasumigaura (Japan): monthly data since 1980

AbstractMicroscopic crustaceans (cladocerans and copepods) and rotifers are the principal zooplankton components of the pelagic food webs in lakes. They play important ecological roles, functioning as essential links between primary producers and planktivorous fish. Individuals zooplankton are important nodes of matter flow in pelagic ecosystems. The zooplankton community structure can provide useful indicators of top–down processes, such as the magnitude or strength of planktivorous predators and the extent of zooplankton grazing. Here we report the abundance of zooplankton taxa (crustaceans and rotifers) that were recorded monthly, from January 1980 to September 2015, at two stations on Lake Kasumigaura, a shallow eutrophic lake that is the second largest lake in Japan. The data include information on 12 copepod species (taxa), 20 cladoceran species (taxa), 40 rotifer species (taxa), and the opossum shrimp (Neomysis intermedia). In the 1980s, the plankton of the lake were characterized by cyanobacterial blooms and the co‐dominance of Bosmina and Diaphanosoma in the summer. In addition, the two cladoceran genera, Daphnia galeata and Chydorus sphaericus were often prominent. The plankton profile changed dramatically in the middle (1997–2004) of the present long‐term monitoring period, when cyanobacteria disappeared and diatoms became dominant even in the summer; concurrently, only the Diaphanosoma cladocerans were evident. However, in the past 10 years, cyanobacterial blooms, the co‐dominance of Bosmina and Diaphanosoma, and D. galeata have re‐emerged. Zooplankton monitoring forms part of the Lake Kasumigaura Long‐Term Environmental Monitoring Program, which has been conducted by the National Institute for Environmental Studies (NIES) since 1977. Data on other planktonic components (phytoplankton and the elements of microbial food webs) noted during monitoring and on primary production were published in Takamura and Nakagawa (Ecol Res 27:8392012a, Ecol Res 27:837 2012b, Ecol Res 31:287 2016). Lake Kasumigaura is a core site of the Japan Long‐term Ecological Research Network, a member of the International Long‐term Ecological Research Network. Our quantitative dataset spanning several decades is unique in terms of the work on lakes and the plankton therein, and is freely available. The dataset has been used in ecological and environmental programs, as well as in studies on lake management.

Modelling the CO2 atmosphere-ocean flux in the upwelling zones using radiative transfer tools

An advanced mathematical model of the radiation forcing on the ocean surface is proposed for the assessment of the CO2 fluxes between atmosphere and ocean boundary in the upwelling zones. Two types of the upwelling are considered: coastal and local in the open ocean that are closely associated with changes in solar irradiance. The proposed model takes into account the maximal number of the carbon fluxes in the upwelling ecosystem considering that in the latter the only original source of energy and matter for all forms of life is the energy of the solar radiation. The vertical structure of the upwelling zone is represented by four levels: the upper mixed layer above the thermocline, the intermediate photic layer below the thermocline, the deep ocean and the near-bottom layer. Peruvian upwelling and typical local upwelling of tropical pelagic region are considered as examples for the model calculations.

Hydrological connectivity determining metacommunity structure of planktonic heterotrophic flagellates

Dispersal potential of organisms is directly related to the hydrological connectivity among habitats. We investigated if the relative role of the environmental and spatial components in structuring the heterotrophic flagellates depends on the degree of hydrological connectivity. Samplings were performed in aquatic environments of the Parana River floodplain, which differ in their degree of connectivity: lotic environments, connected lakes and isolated lakes (temporarily connected to the main river only during floods). We expect that communities in isolated lakes would be more subject to dispersal limitation, while in connected lakes and lotic environments the communities would be regulated mainly by environmental variables (species sorting). We sampled in the pelagic region of 23 environments during the low water period in 2014. The results of pRDA revealed that the contributions of environmental and spatial factors differed between the types of environments. The greater contribution of the environmental variables in structuring the flagellates metacommunity, regardless of the type of environment, may be related to the elevated dispersal capacity of microorganisms. The spatial component was also significant in the isolated lakes. Our results support the idea that microorganism communities are mainly structured by environmental factors, although the spatial component seems important when lakes are isolated.

Drivers of Population Structure of the Bottlenose Dolphin (Tursiops truncatus) in the Eastern Mediterranean Sea

The drivers of population differentiation in oceanic high dispersal organisms, have been crucial for research in evolutionary biology. Adaptation to different environments is commonly invoked as a driver of differ- entiation in the oceans, in alternative to geographic isola- tion. In this study, we investigate the population structure and phylogeography of the bottlenose dolphin (Tursiops truncatus) in the Mediterranean Sea, using microsatellite loci and the entire mtDNA control region. By further comparing the Mediterranean populations with the well described Atlantic populations, we addressed the following hypotheses: (1) bottlenose dolphins show population structure within the environmentally complex Eastern Mediterranean Sea; (2) population structure was gained locally or otherwise results from chance distribution of pre- existing genetic structure; (3) strong demographic varia- tions within the Mediterranean basin have affected genetic variation sufficiently to bias detected patterns of population structure. Our results suggest that bottlenose dolphin ex- hibits population structures that correspond well to the main Mediterranean oceanographic basins. Furthermore, we found evidence for fine scale population division within the Adriatic and the Levantine seas. We further describe for the first time, a distinction between populations in- habiting pelagic and coastal regions within the Mediter- ranean. Phylogeographic analysis suggests that current genetic structure, results mostly from stochastic distribu- tion of Atlantic genetic variation, during a recent post- glacial expansion. Comparison with Atlantic mtDNA haplotypes, further suggest the existence of a metapopulation across North Atlantic/Mediterranean, with pelagic regions acting as source for coastal environments.

Investigating possible causal relations among physical, chemical and biological variables across regions in the Gulf of Maine

We examine potential causal relations between ecosystem variables in four regions of the Gulf of Maine under two major assumptions: (i) a causal cyclic variable will precede, or lead, its effect variable; e.g., a peak (through) in the causal variable will come before a peak (through) in the effect variable. (ii) If physical variables determine regional ecosystem properties, then independent clusters of observations of physical, biological and interaction variables from the same stations will show similar patterns. We use the leading–lagging-strength method to establish leading strength and potential causality, and we use principal component analysis, to establish if regions differ in their ecological characteristics. We found that several relationships for physical and chemical variables were significant, and consistent with “common knowledge” of causal relations. In contrast, relationships that included biological variables differed among regions. In spite of these findings, we found that physical and chemical characteristics of near shore and pelagic regions of the Gulf of Maine translate into unique biological assemblages and unique physical–biological interactions.

Physical controls of oxygen fluxes at pelagic and benthic oxyclines in a lake

We compared oxygen fluxes measured simultaneously at the pelagic and benthic oxycline in a lake and analyze their relation to hydrodynamic forcing conditions. While the mean oxygen fluxes did not differ significantly among both sites, the fluxes were highly variable in time. Short energetic periods contributed disproportionately to the overall oxygen flux above both the benthic and pelagic oxycline. In the pelagic region, mean fluxes across the oxycline were limited by low diffusivities (7 × 10−8 m2 s−1) and were one to two orders of magnitude smaller than fluxes above the oxycline (0.5 and 32 mmol m−2 d−1, respectively). A one‐dimensional transport model was used to estimate sources and sinks of oxygen potentially causing this imbalance. The model results indicate that 92% of dissolved oxygen transported into the oxycline is used by the respiration of organic material imported into the oxycline from the epilimnion; chemical oxygen consumption associated with the upward flux of reduced substances is negligible. Our findings indicate that under such conditions, dissolved oxygen consumption and therewith mineralization within the oxycline can be comparable with the corresponding rates occurring in the sediments of eutrophic lakes with an oxic hypolimnion.

Diet and body mass of ducks in the presence of commercial harvest of brine shrimp cysts in the Great Salt Lake, Utah

Commercial fisheries and waterbirds are often in competition for available resources and may affect each other through limiting available forage for birds or reducing harvest for commercial fisheries. Relatively little is known regarding the wintering population of waterfowl on the hypersaline Great Salt Lake (GSL), Utah, but mid-winter counts indicate waterfowl use the GSL during winter coinciding with a commercial harvest of brine shrimp (Artemia franciscana) cysts (i.e., eggs). We surveyed previously unmonitored pelagic regions of GSL to estimate total wintering populations of waterfowl and collected northern shoveler (Anas clypeata) and green-winged teal (Anas crecca) during 3 nonbreeding seasons, 2009–2012, to examine patterns in diet and body mass of birds using the GSL and estimate the amount of cyst removal by ducks. Our results indicated >200,000 northern shovelers and >150,000 green-winged teal wintered in the GSL and their diets, based on esophageal contents, changed seasonally, likely because of food availability. Esophagi contained predominately wetland plant seeds in fall and spring, whereas GSL invertebrates (i.e., brine shrimp adults and cysts, brine fly [Ephydra spp.] larvae) made up 75–100% aggregate wet weight during winter. Body mass of both species of ducks was greatest in fall and decreased through winter until a partial to full recovery of mass by spring. Though decreases in body mass were positively associated with decreases in GSL cyst abundance, cyst mass in duck diets peaked when cyst abundance was lowest in the GSL. Our results suggest current management of commercial cyst harvest leaves adequate cysts for waterfowl consumption during late winter. Furthermore, estimates of cyst removal from the GSL by ducks suggests they consume <1% of the yearly raw mass removed by ducks and commercial harvest combined, which implies ducks do not have an adverse impact on the commercial cyst harvest. We suggest managers begin incorporating pelagic areas in mid-winter waterfowl surveys to better monitor the relationship between cyst harvest and waterfowl abundance and distributions. © 2014 The Wildlife Society.

Ecology and life-history of Mesonauta festivus: biological traits of a broad ranged and abundant Neotropical cichlid

The cichlid Mesonauta festivus is common and abundant among macrophyte stands along a large geographical range of the Amazonas and Parana-Paraguay basins, in South America. This broad geographical range highlights the species’ dispersion ability, which can be attributed to specific biological and behavioral traits. However, the dispersion ability does not account for the broad geographical range alone, as the species must be able to establish populations in a range of environments, which include marginal areas of large rivers with different water types, floodplain lakes, and small terra-firme streams. In this work we investigated the specie’s ecology, biological traits and behavior in order to understand what and how its traits may have allowed it to attain such broad geographical range and aid in establishing local populations. Regarding its dispersion ability we stress the capability of swimming in the pelagic region, which is remarkable for this species and uncommon among Neotropical cichlids. Its vagility is high even when juveniles are under parental care. Regarding population establishment, the high environmental tolerance stands out, allowing the species to live under strikingly different abiotic conditions. In addition, the small size of first sexual maturation and its capability of spawning along the whole hydrologic cycle (apparently not associated to a specific environmental cue) may also facilitate the establishment of populations into new environments. Moreover, the behavior of mimicking dead leaves, which is mainly performed by juveniles, may lessen predation pressures. Under an eco-evolutionary perspective, the traits highlighted in this work may buffer selective pressures experienced by populations in different biotic and/or abiotic conditions, which may also favor the increasing of the geographical range by allowing the evolutionary lineage to remain similar even in disconnected and/or striking different environments.

Seismic observations from a Yakutat eddy in the northern Gulf of Alaska

AbstractRecent works show that the seismic oceanography technique allows us to relate water column seismic reflections to oceanic finescale structures. In this study, finescale structures of a surface anticyclonic eddy have been unveiled by reprocessing two seismic transects acquired in the northern Gulf of Alaska using an 8 km hydrophone streamer and 6600 cu in linear airgun array in September 2008. The eddy was a typical bowl‐like structure with around 55 km width and 700 m depth. It has two fringes around the eddy base and a spiral arm at the NE edge. The in situ sea surface temperature and salinity data from a shipboard thermosalinograph help to confirm our interpretations of a spiral arm shed from the warm eddy and the entrained cold water from elsewhere. Nearby the eddy and offshore the shelf‐break, there is a strong frontal feature, probably the Alaska Current. The eddy likely formed offshore Yakutat shelf and transported along the offshore shelf‐break by tracking the sea level anomalies. Its equivalent diameter of 65 km was measured using the along‐track altimeter and the seismic constraints. It was comparable with results from the representative conventional algorithms of eddy detection. Geostrophic velocities of the eddy were estimated from the dipping seismic reflections under the assumptions of approximate isopycnals and geostrophic balance. Measured water properties including sea surface temperature, salinity, and chlorophyll revealed that eddy translation transports coastal water to the pelagic regions. Structures synthesized from CTD profiles that sampled an earlier eddy suggest that thin striae around the base might be a common feature in Gulf of Alaska eddies.

Diversity and abundance of rotifers during an annual cycle in the reservoir Valerio Trujano (Tepecoacuilco, Guerrero, Mexico)

We analyzed the diversity and density of monogonont rotifers collected from 10 sites, 5 in the littoral zone and 5 in the pelagic zone, during a 1-year period from March 2010 to April 2011 in the reservoir Valerio Trujano, altitude 820 m a.s.l. (Tepecoacuilco de Trujano, State of Guerrero, Mexico). We also measured physicochemical variables such as temperature, pH, hardness, nitrates, and phosphates. The rotifers were identified to species level and quantified using a Sedgwick-Rafter chamber. We found 62 species belonging to 28 genera and 19 families, representing 22% of total species reported for Mexico. The most abundant species were Brachionus caudatus, B. falcatus, Horaëlla thomassoni, Filinia longiseta, Conochilus dossuarius, and Keratella cochlearis. The highest rotifer densities were those of B. caudatus (around 300 ind. L−1) in June and July. The common phytoplankton genera were Pediastrum, Cosmarium, Closterium, Nitzschia, Suriella, Microcystis, Oscillatoria, and Merismopedia. The species diversity of Rotifera in the reservoir ranged from 1.3 to 3.2 in both the littoral and the pelagic regions. The saprobic index varied from 1.2 to 2 in the pelagic and 1.4 to 2.1 in the littoral region of the reservoir. Nutrient levels and indicator species (Brachionus and Trichocerca) showed that the reservoir, which was nitrogen rather than phosphorus limited, is eutrophic and moderately polluted by organic matter.

Late Triassic compositional changes of aeolian dusts in the pelagic Panthalassa: Response to the continental climatic change

The Carnian pluvial event (CPE) is considered as the drastic environmental change that occurred in the western Tethys during the Carnian (Late Triassic). Although some studies have suggested that the CPE was a global event, no studies have referred to pelagic environment related to the CPE. To reveal the pelagic environment during the Carnian, X-ray absorption fine structure (XAFS) analysis was used to determine the iron (Fe) species in chert samples collected from the Ladinian–Carnian strata in the Tamba–Mino–Ashio Belt, Japan. The X-ray absorption near-edge structure (XANES) analysis shows that all the red cherts contain hematite with a combination of clay minerals, such as illite, chlorite, and smectite. The existence of these minerals is identified using extended XAFS (EXAFS). The XANES analysis of the green and the white cherts does not show any hematite signatures, while the EXAFS analysis indicated increasing amounts of chlorite.The stratigraphic change in the Fe-bearing compositions observed in this study can be divided into three stages: (i) a stage where a relatively stable mineral composition (chlorite+illite+hematite) is observed in cherts deposited during the early Carnian (Julian), (ii) the abrupt change in mineral composition (lack of chlorite and appearance of smectite) observed in the middle area in the studied section and corresponding to the middle part of Carnian (late Julian–early Tuvalian), and (iii) the recovery of the previous stable mineral composition during the late Carnian (Tuvalian). These variations in the mineralogical composition of pelagic cherts can be interpreted to mean that the continental climate conditions changed during the Carnian, and different clay fractions were delivered to the pelagic region. These data, combined with the results of previous studies performed around the continental realm, can be interpreted as follows: (i) there was a relatively stable dry climate before the CPE, (ii) there were increasing amounts of precipitation owing to the CPE, and (iii) the climate returned to dry conditions similar to those prior to the CPE.

Detection and expression of genes for phosphorus metabolism in picocyanobacteria from the Laurentian Great Lakes

The pelagic regions of Lake Superior and eastern Lake Erie (Laurentian Great Lakes) are typically phosphorus (P)-limited environments, and picocyanobacteria of the genus Synechococcus spp. are prominent primary producers during the summer. As a proxy for their utilization of organic P, the expression of two genes, phnD and phoX, was monitored. The phnD gene encodes the phosphonate binding protein of the ABC-type phosphonate transporter, whereas the phoX gene encodes a calcium-dependent alkaline phosphatase. Furthermore, to assess the ability of freshwater Synechococcus spp. to substitute sulfolipids for phospholipids, sqdX gene (cyanobacterial sulfolipid synthase) expression was examined. We employed PCR primers to detect the presence of all three genes in the endemic Synechococcus spp., and RT-PCR assays of cultured freshwater strains and environmental samples to assess the degree of P-stress in the phytoplankton. We show that the phnD gene was constitutively expressed, suggesting that freshwater picocyanobacteria were metabolizing exogenous phosphonate compounds in Lakes Erie and Superior. By contrast, phoX was regulated by P bioavailability. We also provide evidence that sqdX is expressed during increased growth rates in phosphorus-replete conditions, suggesting that sulfolipid synthesis is not a P conservation mechanism for freshwater Synechococcus spp.

Geological setting of basaltic rocks in an accretionary complex, Khangai–Khentei Belt, Mongolia

Abstract We describe the mode of occurrence and geochemical characteristics of basalts, in the Khangai–Khentei belt in Mongolia, overlain by Middle Paleozoic radiolarian chert in an extensive accretionary complex. These basalts are greatly enriched in K, Ti, Fe, P, Rb, Ba, Th, and Nb in comparison to the composition of the mid‐ocean ridge basalts, indicative of within‐plate alkaline type. Ti/Y vs Nb/Y and MnO/TiO2/P2O5 ratios of the basalts also suggest within‐plate affinities. Considering the geochemical characteristics as well as the conformable relationship with the overlying radiolarian chert, the alkaline basalts were clearly not continental but formed a pelagic oceanic island. The mode of occurrence and geochemistry of the basalts show that the alkaline basaltic volcanic activity had taken place to form an oceanic island in the Paleozoic pelagic region sufficiently far from continents to allow radiolarian ooze accumulation.

First occurrence of Norileca triangulata (Crustacea: Isopoda: Cymothoidae) from Indian marine fishes.

An ectoparasitic isopod, Norileca triangulata was found in the branchial cavity of Sardinella gibbosa at Parangipettai coastal waters. The present findings represent the first record of N. triangulata and herein reported. Until now, this species was distributed from Tanimdao Island, Philippines and from Queensland-Eel Reef, Cape York; Michaelmas Cay, near Cairns and Mooloobah, south-eastern Queensland. The range is here extended and now includes to the Southeast coast of India. The materials examined were deposited at the Annamalai University, India (collection Ravichandran). The parasites has been found on 16 out of 16 specimens of S. gibbosa. The prevalence of N. triangulata on S. gibbosa was 7.5% and mean intensity was 1. The host fish length ranges from 140 to 182mm. It is further confirmed that the parasites were specific in the selection of host S. gibbosa. Previously N. triangulata was reported from two hosts Parexocoetus brachypterus. Females of N. triangulata ranges 12-18mm but not found in males. As summarized comparative characteristic feature of two species of parasitic isopods of Norileca indica and N. triangulata. Host species were captured on pelagic region from the coast of Parangipettai. N. triangulata can be distinguished from N. indica by several characters. A related species N. indica has the head to the anterior, and the abdomen facing outwards, pressed against the gill operculum, positioned ventrally in the gill cavity.

Seasonal variation of the phytoplankton community structure in the São João River, Iguaçu National Park, Brazil

The limnological characteristics and the phytoplankton community of the pelagic region of the São João River, tributary of the Iguaçu River, Iguaçu National Park were analyzed from August 2008 to July 2009. 221 taxa were identified and the Bacillariophyceae class was the most representative. Bacillariophyceae and Chrysophyceae were the dominant classes in density and Bacillariophyceae in biovolume. According to the DCA carried out for phytoplankton density and biovolume, significant differences were identified between the periods, and between the sites and study periods, respectively. The highest richness of species reached 40 taxa in September 2008 at station 1. The Shannon-Wiener diversity indexes and evenness, calculated from the density of phytoplankton, were temporally heterogeneous and spatially similar. In general, the significant temporal variations in the composition of the phytoplankton community were due to variations in limnological conditions, mainly temperature, transparency and nutrients. Spatially the structure was more similar due to the proximity among the stations. Moreover, the similarity of the distribution of communities in lotic environments were due to the unidirectional flow.

Physical circulation and spatial exchange dynamics in a shallow floodplain wetland

This paper examines the spatial patterns of water exchange based on water temperature variation between littoral and pelagic zones and compares the patterns in a series of shallow lakes at different water levels. Exchange patterns were assessed by developing isotherms along the transects and estimating the surface energy budget using the vertical temperature profi le and time-series measurements. Our results indicate the presence of density-driven fl ow induced by the differential temperature gradient between littoral areas, which are dominated by either fl oating-leaved or submerged vegetation, and the open pelagic region. Persistent stratifi cation was noted in the narrower lakes, which was thought to be due to the presence of dense submerged vegetation that attenuate wind-driven turbulence. In addition, variation of thermal stratifi cation and mixing dynamics between these lakes at different water levels has corresponding effects on the biological and chemical regimes. The circulation contributes to increased transport of the phosphate that could favour submerged species and subsequently induce shifts of macrophyte community composition. The results of this study have implications for the rehabilitation and management of lake ecosystems.

An oscillating bottom boundary layer connects the littoral and pelagic regions of Lake Opeongo, Canada

The movement of a thermocline can drive strong benthic currents, which can transport nutrients from sediments into the water column via pore water advection or sediment resuspension. We report field observations of near-shore benthic velocities and offshore thermocline movements in Lake Opeongo; a medium-sized lake typical of the Canadian Shield. We find that during large thermocline deflections there are sustained currents &amp;gt;6 cm s−1 in the near-shore benthic layer. The mean current was 1.75 cm s−1 and the maximum current is 10.3 cm s−1. At our site, the net transport is offshore even though the thermocline oscillates up and down so that currents are sometimes upslope and inshore. We estimate the excursion length of a water parcel over the 31-day deployment period, and determine that the mean daily excursion length is 630 m, with the maximum value being 2 km offshore. Given that the south arm of Lake Opeongo is 6 km long and 0.6 km wide, the predicted excursion length of water implies that there is strong connectivity between the sediment in the littoral zone, and the metalimnetic waters offshore. As Lake Opeongo is oligotrophic, any nutrient pulses from the sediment will be quickly taken up by the plankton.

Relationship between phytoplankton composition and environmental factors in the surface waters of southern South China Sea in early summer of 2009

Horizontal distributions of phytoplankton abundance, species composition as well as environmental factors were investigated in the surface waters of southern South China Sea (SCS) in early summer (May 16 to June 7) of 2009. In most areas of southern SCS, the concentrations of nitrogen and phosphorus were very low and DIN/DIP ratios usually were lower than the Redfield N/P Ratio of 16:1. Nitrogen nutrients were significant lower in the upwelling region off Vietnam. A total of 144 taxa of phytoplankton were identified in the study area. The dominant genera were Prorocentrum, Gonyaulax, Gyrodinium, Scrippsiella and Chaetoceros, respectively. Spatial patterns of early-summer phytoplankton abundance and species composition should be mainly controlled by the upwelling off Vietnam and the discharge of Mekong River in the southern SCS. Diatoms dominated in the nutritious Mekong River Estuary or upwelling region off Vietnam; while dinoflagellates dominated in the oligotrophic pelagic region. The canonical correspondence analysis (CCA) indicates that most of diatoms favor higher levels of silicate and phosphorus, as well as lower temperature; while most of dinoflagellates favor the lower silicate and phosphorous and higher temperature. Correlation and CCA results indicate that silicate, nitrate and temperature were the most relevant environmental factors to regulate the horizontal pattern of early-summer phytoplankton in the surface waters of southern SCS.