Pelagic Regions Research Articles

Fluxes of methane, carbon dioxide and nitrous oxide in boreal lakes and potential anthropogenic effects on the aquatic greenhouse gas emissions

We have examined how some major catchment disturbances may affect the aquatic greenhouse gas fluxes in the boreal zone, using gas flux data from studies made in 1994–1999 in the pelagic regions of seven lakes and two reservoirs in Finland. The highest pelagic seasonal average methane (CH 4) emissions were up to 12 mmol m −2 d −1 from eutrophied lakes with agricultural catchments. Nutrient loading increases autochthonous primary production in lakes, promoting oxygen consumption and anaerobic decomposition in the sediments and this can lead to increased CH 4 release from lakes to the atmosphere. The carbon dioxide (CO 2) fluxes were higher from reservoirs and lakes whose catchment areas were rich in peatlands or managed forests, and from eutrophied lakes in comparison to oligotrophic and mesotrophic sites. However, all these sites were net sources of CO 2 to the atmosphere. The pelagic CH 4 emissions were generally lower than those from the littoral zone. The fluxes of nitrous oxide (N 2O) were negligible in the pelagic regions, apparently due to low nitrate inputs and/or low nitrification activity. However, the littoral zone, acting as a buffer for leached nitrogen, did release N 2O. Anthropogenic disturbances of boreal lakes, such as increasing eutrophication, can change the aquatic greenhouse gas balance, but also the gas exchange in the littoral zone should be included in any assessment of the overall effect. It seems that autochthonous and allochthonous carbon sources, which contribute to the CH 4 and CO 2 production in lakes, also have importance in the greenhouse gas emissions from reservoirs.

Stable isotope food web analysis of a large subtropical lake: alternative explanations for 15N enrichment of pelagic vs. littoral fisheries.

The food webs of littoral, pelagic, and littoral-pelagic ecotone (interface) regions of a large subtropical lake were investigated using stable isotope ratio methods, expanding the focus of a previous fish-only study to include other food web components such as primary producers and invertebrates. In these food webs, δC increased ~4o/oo and δN increased ~10o/oo from primary producers to fish. The δN of fish was ~9o/oo in the littoral zone, ~10 o/oo in the ecotone, and ~12o/oo in the pelagic zone. The cross-habitat enrichment in fish N corresponded with both an increase in the size of fish and an increase in the δN of primary consumers (mollusks). Despite larger body size in the pelagic zone, fish in all three habitats appear to occur at the same average trophic level (TL = 4), assuming an enrichment factor of 3.4o/oo per trophic level, and normalizing to the δN of primary consumers.

Environmental control of nitrate uptake in the East China Sea

Abstract Nitrate uptake rate was determined in the East China Sea by a simulated in situ 15 N incubation technique during four cruises representing different seasons in the period from 1993 to 1996. Stations were occupied on an observational line extending from the inner continental shelf to the adjacent oceanic water in the Pacific. The nitrate uptake rate integrated over the euphotic zone was higher on the average on the continental shelf (56 mg N m −2 d −1 ) than at the outer waters of the shelf-break and pelagic regions (21 mg N m −2 d −1 ). Both seasonal and spatial variations were larger on the shelf. Uptake rates were low in summer when the water columns were stratified. Nitrate in the summer surface waters was found to be almost completely depleted by determination with a chemiluminescence technique, and kinetic uptake experiments conducted concurrently suggested severe limitation of uptake due to the low nitrate concentrations. Uptake rates were also low in well-mixed water columns in winter and spring when the water temperature was low. High rates of uptake were observed in a spring bloom as well as at some of the inner-shelf stations in autumn. In the latter case, high phytoplankton biomass was accompanied by detectable amounts of nutrients and by lower salinity at the surface of the stratified water columns. Chlorophyll specific uptake capacity was higher and was significantly correlated with water temperature at a group of stations mostly on the shelf, while the capacity was lower and unaccountably varied at other stations. An empirical prediction model of nitrate uptake from chlorophyll concentration, temperature, light intensity, and nitrate concentration was developed and applied to these data.

Distribution of dissolved organic carbon in Lake Baikal and its watershed

Concentrations of dissolved organic carbon (DOC) in Lake Baikal ranged from 90 to 110 μM C, considerably higher than those in oceanic environments. The DOC concentrations in the epilimnion were higher than those in the hypolimnion. Since particulate organic carbon (POC) concentrations in the pelagic waters of Lake Baikal were <10–40 μM C in the epilimnion and 2–5 μM C in the hypolimnion, DOC constitutes a major component of the organic carbon pool in Lake Baikal, especially in the deep layers. The DOC concentrations downstream of the Barguzin and Selenga Rivers were quite high (400–500 μM C). Probably because of the high concentrations of DOC in these rivers, the DOC levels in Barguzin Bay and offshore at the mouth of the Selenga River were higher than those in the pelagic regions of the central and south basins of Lake Baikal. The relationship between DOC and electric conductivity revealed the transport of DOC from rivers to the pelagic area in Lake Baikal. The spatial distribution of DOC suggested that a major part of DOC in the lake was allochthonous (land-derived).

Natural Radioelement Polonium in Primary Ecological Groups of Black Sea Fishes

Concentrations of 210Po, a natural alpha-radiating radionuclide were determined in various Black Sea fish species sampled along the coastal zone and in the bays of Sevastopol from May to June, 1999. Its value increased in the ecological row from the bottom through the demersal to pelagic region. Among the pelagic species, the Black Sea anchovy and sprat displayed the highest concentrations of 210P. In fishes, the liver was the 210Po accumulating organ.

Geochemistry of Late Permian to Early Triassic pelagic cherts from southwest Japan: implications for an oceanic redox change

Major, trace, and rare earth element abundances were determined for the southwestern Japanese pelagic chert sequence from the early Late Permian to early Early Triassic to investigate a redox change in deep-sea pelagic environments before and at the Permo–Triassic boundary (PTB) (251 Ma). The sequence was primarily deposited in the deep-sea of the superocean Panthalassa, and then was accreted to Japan in the Middle Jurassic. A remarkable lithostratigraphic change from red chert to siliceous∼carbonaceous claystone through gray chert is observed in this sequence. Constituent elements for these sedimentary rocks are essentially derived from two sources: (1) an ancient seawater via biogenic (mainly radiolarian), hydrothermal, and authigenic materials and (2) an average shale-like terrigenous material. The present measurement demonstrates significant stratigraphic changes of the Ce/Ce non-ter* value (estimated Ce anomaly value of non-terrigenous component, recalculated by subtraction of terrigenous REEs from bulk REEs) and the (Mn/TE) sample/(Mn/TE) PAAS value (excess Mn component other than terrigenous one; TE=terrigenous elements including Ti, Al, Nb, Hf, Th; PAAS=Post-Archean Average Australian Shale) in the Permian chert. The Ce/Ce non-ter* values increase from <0.2 to 1 and the (Mn/TE) sample/(Mn/TE) PAAS values decrease up-section, suggesting that the redox condition of deep-sea open-ocean changed from oxic to suboxic in an interval of approximately 10 Myr. The (∑Fe/TE) sample/(∑Fe/TE) PAAS and (Mn/TE) sample/(Mn/TE) PAAS values of carbonaceous claystone near or at the PTB are less than unity, suggesting that reductive dissolution of iron and manganese occurred under an anoxic condition. This supports the idea of the PTB oceanic anoxia in the superocean Panthalassa. The present data suggest that the anoxic condition prevailed in the deep-sea pelagic regions for an extremely long period, much more than 10 Myr, from the middle Late Permian to early Early Triassic. This long-term development of widespread oceanic anoxia may have been linked to the greatest mass extinction of the Phanerozoic.

Distribution of CH 4 in the north basin of Lake Biwa and tributary rivers

To determine the origin of CH4, the vertical distribution of CH4 around the thermocline in the north basin of Lake Biwa and the horizontal distribution in the Yasu and Ado rivers were measured. In 1995, CH4 concentrations (ranging from 200 to 1000 nM) exceeding the saturation level were observed just above or in the thermocline in the pelagic region off the Yasu River. These values were higher than those just below the thermocline and in the hypolimnion. CH4 concentrations in the mouths of the Yasu and Ado rivers were much higher than at other stations, around 2500 and 2000 nM, respectively. Due to the drought in 1994 when there was no water from the Yasu River flowing into the north basin, CH4 concentration just above or in the thermocline in the pelagic region off the Yasu River ranged from 49 to 74 nM. It is thus concluded that the high level of CH4 observed in the mouth of the Yasu River is one of the sources of the high CH4 concentrations in the pelagic region off the Yasu River. On the other hand, the CH4 concentration in the pelagic region off the Ado River was about 50 nM. It is thought that water flowing in from the Ado River diffused readily into the lake water because the depth of the lake in the region off the Ado River declines steeply. The maximum concentration of CH4 in the river mouths indicates that these areas are important sites for clarifying the mechanism of the decrease in dissolved oxygen in Lake Biwa and the Yodo River watershed.

Nutrient effects on producers and consumers in the littoral plankton and periphyton of a subtropical lake

An in situ mesocosm experiment was conducted to evaluate the effects of increased nutrient loading on both producers and micro-consumers (protozoa, rotifers, and micro-crustacea) in the nutrient-deficient littoral zone of a subtropical lake. Areas of the natural community were enclosed in replicate 1.2 m 2 transparent tubes and treated with nitrogen, phosphorus, or nitrogen plus phosphorus over 28 days. Responses of six distinct community components (floating surface periphyton mats, phytoplankton, epiphyton associated with Utricularia, epiphyton on live and dead stems of Eleocharis, and a benthic algal mat) were evaluated and compared to non-enriched controls. All treatments were carried out in triplicate and nutrient inputs simulated those measured in the lake's more eutrophic pelagic region. Nutrient additions stimulated growth in a consistent manner across both habitats and trophic levels. Although there was a limited number of responses to addition of nitrogen or phosphorus alone, the overwhelming response was increased biomass when the two nutrients were added together. One community component that did not display an increase in algal biomass in response to nutrient addition was the benthic algal mat. Nevertheless, benthic consumers increased significantly in all nutrient-addition treatments. This may have been a response to the rain of particulate material stimulated by nutrients higher up in the water column. To our knowledge, this is the first study to evaluate nutrient effects on littoral micro-consumers in a subtropical setting; results generally were consistent with those obtained in temperate lakes.

The methane cycle in the epilimnion of Lake Constance

During summer, methane concentrations measured in the surface water layers of Lake Constance were oversaturated by as much as forty times. Although methane concentrations were less than saturated in the upper water column during the winter of 1994/95, the concentrations increased to more than 2000 nl/l (90 nM) in July. The observed spatiotemporal distribution patterns strongly suggest biogenic production of methane in oxic epilimnetic water layers whereas advective transport from the littoral to the pelagic region or from allochthonous river inputs had apparently less impact on pelagic methane concentrations. Statistically significant correlations of methane concentrations with biomass parameters (e.g. phaeophytin, particulate phosphorus) hint at lake-snow as a potential site of epilimnetic in situ production of methane. In contrast, field data did not provide compelling evidence of methanogenesis in the intestinal tract of zooplankton. In diel cycles, high nocturnal methane concentrations alternate with low concentrations of 13 C-enriched methane during the daytime. This happens due to the phenomenon of oscillating oxygen concentrations in littoral water and sediment, which is affected by the antagonism of photosynthetic and respiratory activity. Due to the observed oversaturation, epilimnetic methane levels in summer and methane concentrations below saturation in winter, Lake Constance provided a sink for methane from January to April, but for the rest of the year a considerable methane flux to the atmosphere was calculated.

Geochemistry of Rare Earth Elements and Thorium in Sediments and Ferromanganese Nodules of the Atlantic Ocean

The behavior of rare earth elements (REE) and Th is studied along the west–east transect at 22°N across the Atlantic Ocean. It is shown that both REE and Th contents, relative to Al (the most lithogenic element), increase toward the pelagic region. The increasing trend becomes more complicated due to variations in the content of biogenic carbonate that serves as a diluting component in sediments. The REE composition varies symmetrically relative to the Mid-Atlantic Ridge (MAR) emphasizing a weak hydrothermal influence on sediments of the ridge axis, although the well-known criteria for hydrothermal contribution, such as Al/(Al + Mn + Fe) and (Fe + Mn)/Ti, do not reach critical values. Variations in the REE content and composition allowed us to distinguish the following five sediment zones in the transect: (I) terrigenous sediments of the Nares abyssal plain; (II) pelagic sediments of the North American Basin; (III) carbonate ooze of the MAR axis; (IV) pelagic sediments of the Canary Basin; and (V) terrigenous clay and calcareous mud of the African continental slope and slope base. Ferromanganese nodules of the hydrogenetic type with extremely high Ce (up to 1801 ppm) and Th (up to 138 ppm) contents occur in pelagic sediments. It is ascertained that P, REE, and Th concentrations depend on Fe content in Atlantic sediments. Therefore, one can suggest that only a minor amount of phosphorus is bound in bone debris. The low concentration of bone debris phosphorus is a result of relatively high sedimentation rates in the Atlantic, as compared with those in pelagic regions of the Pacific.

DISSOLVED ORGANIC CARBON IN TWO BRAZILIAN COASTAL LAGOONS: SOURCES AND UTILIZATION FOR HETEROTROPHIC BACTERIA

We evaluated the bacterial utilization of dissolved organic matter (DOM) of a leachate from emergent macrophytc, Typha domiingensis Pers, dominant in two coastal lagoons of different dissolved organic carbon (DOC) concentrations, in the north of Rio de Janeiro State, Brazil. The leachate was added to microcosms with pelagic water Iron the two lagoons and inoculated with natural bacteria. Control experiments were performed using only pelagic fi1tered water added to inocculum. Bacterial production, density and biomass were measured every 24 hours, during five days. Bacterial production and biomass were significantly higher in experiments with leachate addition, in both lagoons, in comparison to the controls. Those results indicate that DOM from the macrophyte has a positive ini1ucncc on bacterial growth. Normalized results also indicate it is not a matter of quantity, but the quality of DOM is better utilized than the DOM Iron pelagic region.

Lotic–lentic linkages associated with Lake Okeechobee, Florida

The hydroscape of south Florida has been severely altered over the past 50 y by the construction of canals, drainage ditches, levees, water control structures, and pump stations. This alteration has included, in part, channelization of the Kissimmee River, construction of a dike around Lake Okeechobee, increased ditching of pastures for drainage throughout the lake’s watershed, and unnatural connections from Lake Okeechobee to estuaries on the east and west coasts of Florida. Collectively, these processes have reduced the retention of flow in the lake’s watershed, restricted the opportunity for nutrients to be assimilated on the landscape and, in turn, increased the rate of eutrophication in Lake Okeechobee. This paper reviews the history of these changes and current plans for system restoration. The fate and transport of P is used to illustrate how hydrologic changes have influenced the system’s ecology. Improved drainage has been a key factor allowing for the expansion of dairy and beef cattle operations. This, in turn, has increased agricultural runoff to Lake Okeechobee. Best management practices have helped reduce total P (TP) loads to the lake. However, TP concentrations in the pelagic region of the lake have not declined, presumably because of internal loading. Watershed and water-quality models are being used to examine the effect of changing land use in the watershed on the lake’s condition. A number of large, multiagency projects are being developed in south Florida, with the goal to develop a hydrologic system that meets the multiple needs of agriculture, urban development, and the environment. Aquatic linkages, including lotic–lentic, lotic–lentic–estuarine, and lotic–wetland, are critical aspects of these projects.BRIDGES is a recurring feature of J-NABS intended to provide a forum for the interchange of ideas and information between basic and applied researchers in benthic science. Articles in this series will focus on topical research areas and linkages between basic and applied aspects of research, monitoring policy, and education. Readers with ideas for topics should contact Associate Editors, Nick Aumen and Marty Gurtz.Benthic scientists usually focus their research within 1 type of aquatic ecosystem. However, junctions between lotic and lentic ecosystems are often physically, chemically, and biologically complex, and are important gateways to the matter and energy dynamics within either or both of the adjoining systems. A symposium on Lotic–Lentic Interactions, held on 5 June 1998 at the 46th Annual NABS Meeting in Charlottetown, Prince Edward Island, highlighted the research being done on this topic. Two of the papers from the symposium dealt with restoration and management projects and, as such, link basic and applied benthic research. These 2 papers are presented below. A. D. Steinman and B. H. Rosen present an overview of restoration efforts for the Kissimmee River/Lake Okeechobee hydroscape in south Florida in the context of historical land-use changes in the watershed. These changes have accelerated P loading and eutrophication in Lake Okeechobee. The effects are demonstrated by watershed and water-quality models. E. P. Benenati et al. examine linkages between the Lake Powell Reservoir and the downstream Colorado River. These authors show that the Colorado River benthic community is strongly affected by discharges from the Lake Powell Reservoir, caused by snowmelt pulses or experimental flooding. The 2 papers illustrate the need to carefully consider adjoining ecosystems in studies of lotic and lentic systems, and in management and restoration scenarios.A. HersheyA. D. SteinmanNick Aumen, [email protected]Marty Gurtz, [email protected]Co-editors

Co-evolution of movement behaviours by tropical pelagic predatory fishes in response to prey environment: a simulation model

Predatory fishes, such as tunas, billfishes, and sharks, coexist in pelagic regions of the tropical oceans. In situ experiments have revealed horizontal and vertical movement patterns for different pelagic species, but the influence of the biotic environment on movement behaviour has not been studied. In this paper, we propose a simple model in which the movement behaviour of these fishes is driven entirely by the biotic environment, without implementing physiological constraints. We explore this concept via computer simulations based on the Latent Energy Environments model [Menczer, F., Belew, R.K., 1996a. From complex environments to complex behaviors. Adapt. Behav. 4(3/4), 317–63]. In our model, multiple behaviours for artificial fishes evolve in a three-dimensional environment where spatial and temporal distributions of prey are patterned after hydroacoustic data taken during ultrasonic telemetry experiments on tunas in the open ocean in French Polynesia. Interactions among individuals are modeled through their shared prey resources. Movement patterns of the adapted individuals are analyzed to: (i) compare artificial individuals with real fishes (three species of tuna, three species of billfishes, and one species of shark) observed by ultrasonic telemetry; and (ii) examine how the artificial fishes exploit their environment. Most of the individuals evolved vertical patterns virtually identical to those exhibited by fishes in the wild. The agreement between our simple model and the ethological data validates the use of computational models for studies of the characteristics of multiple species inhabiting a common ecosystem.

Contribution of Gammarus lacustris to phosphorus recycling in a fishless alpine lake

We estimated the net P transport by Gammarus lacustris from the benthic to pelagic regions of a fishless alpine lake and compared it with P regeneration by the entire plankton community. Gammarus lacustris released between 5.2 and 18.1 ng P·L-1·h-1 (adults only and adults plus immatures, respectively) in the pelagic region during nighttime vertical migration. Additional P released into and removed from the water column due to predation on zooplankton was estimated at 1.87 and 2.3 ng P·L-1·h-1, respectively. The net daily regeneration of 52.2-181.4 ng P·L-1·day-1 by the G. lacustris population represented 9.5-32.9% of the total P regenerated by the planktonic community. The majority of the P released by G. lacustris represents "new" P to the pelagic zone because it originated in sediments. We conclude that G. lacustris can represent an important link in benthic-pelagic coupling in oligotrophic mountain lakes.

Filtering Impacts of an Introduced Bivalve ( Dreissena polymorpha ) in a Shallow Lake: Application of a Hydrodynamic Model

Nonindigenous species may exert strong effects on ecosystem structure and function. The zebra mussel (Dreissena polymorpha) has been attributed with profound changes in invaded ecosystems across eastern North America. We explored vertical profiles of water flow velocity and chlorophyll a concentration in western Lake Erie, over rocky substrates encrusted with Dreissena, to assess the extent to which mussels influence coupling between benthic and pelagic regions of the lake. Flow velocity was always low at surveyed sites (less than or equal to 2.9 cm s-1) and declined in direct proximity to the lakebed. Mean chlorophyll a concentration was also low (less than 5μg L-1) at all sites and depths. Chlorophyll a concentration was positively correlated with distance above lakebed and was lowest (0.3μg L-1) directly adjacent to the lakebed. Spatial patterns of zooplankton grazers could not explain observed vertical gradients in chlorophyll concentration. Hydrodynamic modeling revealed that filtering effects of Dreissena in a nonstratified, shallow basin depend mainly on upstream chlorophyll concentration, intensity of turbulent diffusion, feeding efficiency of the mussel colony, and the distance downstream from the leading edge of the mussel colony. In contrast to widespread perceptions that molluscs reduce phytoplankton concentration only adjacent to the lakebed, modeling scenarios indicated that depletion occurs throughout the water column. Depletion was, however, inversely proportional to distance above the lakebed. Simulation results are consistent with field-based observations made in shallow water habitats populated by large Dreissena populations in the Great Lakes and elsewhere. Results from this study indicate that zebra mussels strongly enhance coupling between pelagic and benthic regions in shallow lakes. Enhanced coupling between these regions explains, in part, high population densities of Dreissena and of many benthic invertebrates in ecosystems invaded by zebra mussels.

Littoral periphyton responses to nitrogen and phosphorus: an experimental study in a subtropical lake

Portions of the natural littoral community in a subtropical lake were enclosed in mesocosms and subjected to high rates of nitrogen (N) and/or phosphorus (P) loading over a 28-day period. Changes in periphyton structure and function were compared with untreated controls to evaluate limiting resource effects. Mesocosms that received only P displayed minor increases in whole-community metabolism, measured by in situ oxygen evolution during mid-day hours, and small increases in the chlorophyll a (CHLA) content of surface periphyton mats. Mesocosms that received only N displayed significant increases in the CHLA content of epiphyton growing on submerged stems of Eleocharis. The most striking treatment effects were observed when both P and N were added. In that case, there were highly significant increases in the CHLA content of surface periphyton mats, epiphyton associated with submerged Utricularia, and periphyton growing on artificial substrata. There also were increases in the rates of phytoplankton carbon uptake and whole-community metabolism. These results indicate that periphyton and phytoplankton were co-limited by P and N, in contrast to previous findings of strong P limitation in more pristine regions of this marsh, and in the nearby Florida Everglades. The community responses in the present study may reflect early eutrophication effects, because the study site was located in close proximity to the lake's eutrophic pelagic region.

Localized Changes in Transparency Linked to Mud Sediment Expansion in Lake Okeechobee, Florida: Ecological and Management Implications

ABSTRACT During the last decade, water quality monitoring data have indicated declines in Secchi transparency in the southwestern pelagic region of Lake Okeechobee. Sampling stations that previously displayed intra-annual variation in Secchi depths from 0.2 to 2.7 m now display Secchi depths that never exceed 1.0 m. The historical changes in transparency occurred abruptly, and the recent low transparencies have been very stable. Ratios of Secchi/total depth indicate that light often may have reached the lake bottom (ratios > 0.5, and as high as 1.0) prior to the new stable state, but never reached the bottom afterwards (ratios <0.3). Secchi depth vs. chorophyll a relationships also have changed in the southwestern pelagic region. Stations previously displayed significant inverse log relationships between the two variables, similar to what is observed today at near-shore, sand-bottomed sites in Lake Okeechobee. In recent years, there has been no relationship between Secchi depth and chlorophyll a. This is similar to the mud-bottomed central pelagic region, where abiotic turbidity controls light attenuation. One explanation for the sudden changes in Secchi transparency is migration of mud sediments from mid-lake towards the southwestern shore. A comparison of sediment maps constructed in 1975 and 1988 indicates that the area of lake bottom covered by mud sediment increased from approximately 30 to 44% over that period. The major direction of expansion was to the southwest, and the postulated movement coincided closely with dates when Secchi declines were observed at water quality monitoring stations. The declines in transparency that have occurred in the southwest near-shore region of Lake Okeechobee may have important ecological and societal implications. This near-shore region is the primary foraging and nesting habitat for commercial and sport fish, wading birds, and migratory water fowl. Losses of submerged plant communities due to reduced light penetration may adversely affect these communities. In addition to a present goal of external P load reduction, lake managers should give consideration to in-lake methods that might prevent further water quality degradation in near-shore regions. Because water depth can act synergistically with turbidity to bring about the changes described above, one option is a substantial draw-down of the lake, followed by actions to prevent the prolonged high lake stages that recently have led to submerged plant declines. Selective dredging of sediments from the southwest region of the lake might also be considered, although this would likely be only a short-term solution to the water quality problems, given the remaining large source of mud at the center of the lake.

Age and growth of Electrona antarctica (Pisces: Myctophidae), the dominant mesopelagic fish of the Southern Ocean

Numerically and in biomass, the lanternfish Electrona antarctica is the dominant fish in the vast pelagic region of the Southern Ocean bounded on the north by the Antarctic Convergence and in the south by the Antarctic continental shelf. It is an important krill predator, and in turn is important in the diets of flighted and swimming seabirds. Further, it is the southernmost and coldest-dwelling representative of the globally distributed fish family Myctophidae. The present study was undertaken to estimate the species' growth rate and average life span, to incorporate the information in a basic energy budget, and to compare the growth of E. antarctica with more northerly confamilials. Fishes were aged using primary growth increments that were resolved on sagittal otoliths using three sequential techniques: thin-section grinding and polishing, etching, and scanning electron microscopy (SEM). Based on increment width (0.8 to 1.2 μm), continuity, and previous studies on confamilials, the microincrements were assumed to be deposited on a daily basis. Montages of SEM photomicrographs were constructed for each sagitta to allow the daily rings to be counted over the entire life span of 31 individuals representing the entire size range of the species. Results suggest a larval stage of 30 to 47 d and a maximum life span of 3.5 yr, with females growing faster than males in the last 1.5 yr of life and reaching a larger maximum size. Construction of a simple energy budget using the best information available suggests that a surplus of energy is available to support the observed growth rates (0.05 to 0.07 mm d−1). The results of the present study contrast markedly with previous estimates of an 8 to 11 yr maximum age for E. antarctica. These results provide important data addressing the ecology and population dynamics of the pelagic Antarctic ecosystem. E. antarctica is the end-member species in the continuum of vertically migrating myctophids that extend from the equator to the polar circle. Its growth rate is consonant with that of all other myctophid species examined using primary growth increments to determine age. The present study, in conjunction with earlier studies, suggests that growth rates of mesopelagic species are far higher than previously thought.

Effect of temperature on inter- and intraspecific isolates of Urotricha (Prostomatida, Ciliophora)

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 15:285-291 (1998) - doi:10.3354/ame015285 Effect of temperature on inter- and intraspecific isolates of Urotricha (Prostomatida, Ciliophora) Thomas Weisse1,*, David J. S. Montagnes2 1Max Planck Institute for Limnology, PO Box 165, D-24302 Plön, Germany 2Port Erin Marine Laboratory, University of Liverpool, Port Erin, Isle of Man IM9 6JA, United Kingdom *Present address: Institute of Limnology of the Austrian Academy of Sciences, Gaisberg 116, A-5310 Mondsee, Austria. E-mail: thomas.weisse@oeaw.ac.at ABSTRACT: Ecological models often presume that all members of a functional guild, e.g. herbivorous ciliates, respond identically to temperature changes. To test this general assumption, we investigated if planktonic ciliates within the genus Urotricha exhibit distinct inter- and intraspecific responses to temperature. We examined the response of growth rate, cell volume, and production to changing temperature, using 6 temperatures between 5 and 30°C. This experiment made 3 comparisons using: (1) different species isolated from the pelagic and littoral regions of the same lake; (2) different clones of the same species isolated from lakes of the similar trophic status, but different latitudes; and (3) different clones of the same species isolated from the pelagic region of a single lake. Using ANCOVA and ANOVA procedures (a = 0.05) to examine the data we demonstrated that: (1) ciliate species within a single genus may exhibit distinct responses to temperature, suggesting that treating ciliates as a single functional group is an oversimplification; (2) clones isolated from a laboratory culture of a single species, isolated from a single location, differ only to a small extent; and (3) clones isolated from different localities, but belonging to the same species differ considerably in their responses. Our data indicate that (1) temperature regimes may be an environmental niche that separates species and clones and (2) even where apparent morphological and molecular differences do not distinguish taxa, functional differences may still exist. KEY WORDS: Ciliate · Temperature response · Clonal variation · Growth rate · Production · Cell volume Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 15, No. 3. Publication date: August 31, 1998 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 1998 Inter-Research.

Changes in Phytoplankton Size-class Abundance and Species Composition Coinciding with Changes in Water Chemistry and Zooplankton Community Structure of Lake Michigan, 1983 to 1992

Phytoplankton and zooplankton were collected at offshore sites of Lake Michigan during 37 cruises in the spring and summer from 1983 to 1992. For the period, 39 common phytoplankton species accounted for 96.0% of the total abundance and 85.6% of the biomass. Over the 10-year study, the lake-wide average spring and summer phytoplankton biomass in the pelagic waters ranged from 0.27 to 1.2 g/m 3 (mean ± S.E. = 0.54 ± 0.03 g/m 3) and phytoplankton abundance ranged from 5,132 to 39,780 cells/mL (mean ± S.E. = 18,291 ± 822 cells/mL). Mesotrophic diatoms accounted for 47.2% of the total phytoplankton biomass. The lack of a trend in the ratio of mesotrophic to eutrophic diatom indicator species suggested that no change in trophic status of the pelagic region occurred during the 1983 to 92 period; that is, the water quality of the offshore of Lake Michigan did not change. A year-to-year shift in dominance from one mesotrophic diatom species to another was evident. Over the 10-year period, flagellates accounted for 52.2% of the summer phytoplankton biomass. The data support the concept of a shift in summer species composition away from blue-green algae dominance to flagellates prior to 1982. However after 1987, the relative importance of the > 70 μm size class increased to over 21% of the phytoplankton abundance in the spring and over 55% in the summer. In particular, the colonial blue-greens, Anacystis and Aphanothece, became substantially more prevalent during the summer, while the flagellates, Chromulina and Ochromonas, decreased in abundance. The changes in the relative abundance of phytoplankton size classes and species composition were examined to determine if they correlated with either the top-down mediated changes in the zooplankton community during the 1980s and 1990s or with any bottom-up variability in nutrient chemistry. Canonical correspondence analysis (CCA) suggested a significant portion of the variability of the spring and summer phytoplankton size classes from 1983–1992 (88.0% and 99.2%, respectively) was explained by environmental axis 1. Primary determinants of relative abundance of phytoplankton size class included nutrients (silica, total phosphorus, and N:P ratio) and abundance of some species of zooplankton. Specifically, Diaptomus minutus, D. ashlandi, D. sicilis, and Daphnia galeata mendotae were inversely correlated with the 0 to 10 μm size class, which we interpret as a grazing effect on phytoplankton size structure, and positively correlated with an increase in abundance of the > 70 μm size class of phytoplankton. CCA suggested that top-down and bottom-up effects were affecting phytoplankton size composition and abundance simultaneously.