Net heterotrophy in Faroe Islands clear-water lakes: causes and consequences for bacterioplankton and phytoplankton

1. Five oligotrophic clear‐water lakes on the Faroe Islands were studied during August 2000. Algal and bacterial production rates, community respiration, and CO₂ saturation were determined. In addition, we examined the plankton community composition (phytoplankton and heterotrophic nanoflagellates) and measured the grazing pressure exerted by common mixotrophic species on bacteria. 2. High respiration to primary production (6.6–33.2) and supersaturation of CO₂ (830–2140 μatm) implied that the lakes were net heterotrophic and that the pelagic heterotrophic plankton were subsidised by allochthonous organic carbon. However, in spite of the apparent high level of net heterotrophy, primary production exceeded bacterial production and the food base for higher trophic levels appeared to be mainly autotrophic. 3. We suggest that the observed net heterotrophy in these lakes was a result of the oligotrophic conditions and hence low primary production in combination with an input of allochthonous C with a relatively high availability. 4. Mixotrophic phytoplankton (Cryptomonas spp., Dinobryon spp. and flagellates cf. Ochromonas spp.) constituted a large percentage of the plankton community (17–83%), possibly as a result of their capacity to exploit bacteria as a means of acquiring nutrients in these nutrient poor systems.     

Scaling of Pelagic Metabolism to Size,Trophy and Forest Cover in Small Danish Lakes

ABSTRACT We tested whether pelagic light and nutrient availability, metabolism, organic pools and CO₂-supersaturation were related to lake size and surrounding forest cover in late summer–autumn measurements among 64 small (0.02–20 ha), shallow seepage lakes located in nutrient-rich, calcareous moraine soils in North Zealand, Denmark. We found a strong implicit scaling to lake size as light availability increased significantly with lake size while nutrient availability, phytoplankton biomass and dissolved organic matter declined. Forest lakes had significantly stronger net heterotrophic traits than open lakes as higher values were observed for light attenuation above and in the water, dissolved organic matter, pelagic community respiration (R) relative to maximum gross primary production (R/GPP) and CO₂-supersaturation. Total-phosphorus was the main predictor of phytoplankton biomass (Chl) despite a much weaker relationship than observed in previous studies of larger lakes. Maximum gross primary production increased with algal biomass and decreased with dissolved organic matter, whereas community respiration increased with dissolved organic matter and particularly with gross primary production. These results suggest that exogenous organic matter supplements primary production as an energy source to heterotrophs in these small lakes, and particularly so in forest lakes experiencing substantial shading from the forest and dissolved humic material. This suggestion is supported by 20–30-fold CO₂ supersaturation in the surface water of the smallest forest lakes and more than sixfold supersaturation in 75% of all measurements making these lakes among the most supersaturated temperate lakes examined so far.     

Inorganic carbon promotes photosynthesis,growth, and maximum biomass of phytoplankton in eutrophic water bodies

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Net Heterotrophy in Small Danish Lakes: A Widespread Feature Over Gradients in Trophic Status and Land Cover

Nineteen small lakes located in open landscapes or deciduous forests in nutrient-rich calcareous moraines in North Zealand, Denmark, were all net heterotrophic having negative net ecosystem production and predominant CO₂ supersaturation and O₂ undersaturation of lake waters. Forest lakes were poorer in nutrients, phytoplankton, and primary production, but richer in dissolved organic matter and CO₂ than open lakes with more light available. The modeled annual balance between gross primary production and community respiration (GPP/R _COM) averaged 0.60 in forest lakes and 0.76 in open lakes and the ratio increased significantly with phosphorus concentration and phytoplankton biomass but decreased with colored dissolved organic matter. The negative daily rates of ecosystem production resembled estimates of oxygen uptake from the atmosphere to the lakes, whereas estimates of CO₂ emission were 7.2-fold higher. Although CO₂-rich groundwater and anaerobic respiration support greater molar release of CO₂ than uptake of O₂, we suggest CO₂ emission is overestimated. Possible explanations include CO₂ enrichment of the air film above small wind-sheltered lakes. The observed metabolism and gas exchange show that exogenous organic matter is an important supplementary energy source to community respiration in these small lakes and that forest lakes, in addition, experience pronounced light attenuation from trees and dissolved colored organic matter constraining primary production. Kaj Sand-Jensen (KSJ) formulated the original research idea and designed the study. Data analysis (ie. calculations, statistics and figures) was performed by Peter A. Staehr (PAS). The text was for the most part written by KSJ, although with contributions by PAS, especially in describing the applied methods and data analysis.     

Carbon dioxide supersaturation promotes primary production in lakes

A majority of the world's lakes are supersaturated with respect to carbon dioxide (CO(2) ). By experimental manipulation of the CO(2) concentration in supersaturated boreal lakes, we demonstrate that phytoplankton primary production was up to 10 times higher in supersaturated lake water in comparison with water with CO(2) at equilibrium concentrations and that CO(2) , together with nutrients, explained most of the variation in pelagic primary production and phytoplankton biomass over a wide variety of unproductive lakes. These results suggest that phytoplankton can be co-limited by CO(2) and nutrients in unproductive lakes. As import of terrestrial organic carbon and its subsequent microbial mineralisation in lakes is a driving force of CO(2) -supersaturation our results suggest that lake productivity and carbon cycling may respond to variations in terrestrial organic carbon export, (e.g. caused by land use or climate change) in ways not described before.     

Freshwater biota and rising pCO2?

Rising atmospheric carbon dioxide (CO₂) has caused a suite of environmental issues, however, little is known about how the partial pressure of CO₂ (pCO₂) in freshwater will be affected by climate change. Freshwater pCO₂ varies across systems and is controlled by a diverse array of factors, making it difficult to make predictions about future levels of pCO₂. Recent evidence suggests that increasing levels of atmospheric CO₂ may directly increase freshwater pCO₂ levels in lakes, but rising atmospheric CO₂ may also indirectly impact freshwater pCO₂ levels in a variety of systems by affecting other contributing factors such as soil respiration, terrestrial productivity and climate regimes. Although future freshwater pCO₂ levels remain uncertain, studies have considered the potential impacts of changes to pCO₂ levels on freshwater biota. Studies to date have focused on impacts of elevated pCO₂ on plankton and macrophytes, and have shown that phytoplankton nutritional quality is reduced, plankton community structure is altered, photosynthesis rates increase and macrophyte distribution shifts with increasing pCO₂. However, a number of key knowledge gaps remain and gaining a better understanding of how freshwater pCO₂ levels are regulated and how these levels may impact biota, will be important for predicting future responses to climate change.     

Coupling dissolved organic carbon,CO2 and productivity in boreal lakes

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The catchment and climate regulation of pCO2 in boreal lakes

The regulation of surface water pCO₂ was studied in a set of 33 unproductive boreal lakes of different humic content, situated along a latitudinal gradient (57°N to 64°N) in Sweden. The lakes were sampled four times during one year, and analyzed on a wide variety of water chemistry parameters. With only one exception, all lakes were supersaturated with CO₂ with respect to the atmosphere at all sampling occasions. pCO₂ was closely related to the DOC concentration in lakes, which in turn was mainly regulated by catchment characteristics. This pattern was similar along the latitudinal gradient and at different seasons of the year, indicating that it is valid for a variety of climatic conditions within the boreal forest zone. We suggest that landscape characteristics determine the accumulation and subsequent supply of allochthonous organic matter from boreal catchments to lakes, which in turn results in boreal lakes becoming net sources of atmospheric CO₂.     

Predictable ecological response to rising CO2 of a community of marine phytoplankton

Rising atmospheric CO₂ and ocean acidification are fundamentally altering conditions for life of all marine organisms, including phytoplankton. Differences in CO₂ related physiology between major phytoplankton taxa lead to differences in their ability to take up and utilize CO₂. These differences may cause predictable shifts in the composition of marine phytoplankton communities in response to rising atmospheric CO₂. We report an experiment in which seven species of marine phytoplankton, belonging to four major taxonomic groups (cyanobacteria, chlorophytes, diatoms, and coccolithophores), were grown at both ambient (500 μatm) and future (1,000 μatm) CO₂ levels. These phytoplankton were grown as individual species, as cultures of pairs of species and as a community assemblage of all seven species in two culture regimes (high‐nitrogen batch cultures and lower‐nitrogen semicontinuous cultures, although not under nitrogen limitation). All phytoplankton species tested in this study increased their growth rates under elevated CO₂ independent of the culture regime. We also find that, despite species‐specific variation in growth response to high CO₂, the identity of major taxonomic groups provides a good prediction of changes in population growth and competitive ability under high CO₂. The CO₂‐induced growth response is a good predictor of CO₂‐induced changes in competition (R² > .93) and community composition (R² > .73). This study suggests that it may be possible to infer how marine phytoplankton communities respond to rising CO₂ levels from the knowledge of the physiology of major taxonomic groups, but that these predictions may require further characterization of these traits across a diversity of growth conditions. These findings must be validated in the context of limitation by other nutrients. Also, in natural communities of phytoplankton, numerous other factors that may all respond to changes in CO2, including nitrogen fixation, grazing, and variation in the limiting resource will likely complicate this prediction.     

INORGANIC CARBON UTILIZATION BY ROSS SEA PHYTOPLANKTON ACROSS NATURAL AND EXPERIMENTAL CO2 GRADIENTS1

We present results from a field study of inorganic carbon (C) acquisition by Ross Sea phytoplankton during Phaeocystis‐dominated early season blooms. Isotope disequilibrium experiments revealed that HCO₃^? was the primary inorganic C source for photosynthesis in all phytoplankton assemblages. From these experiments, we also derived relative enhancement factors for HCO₃^?/CO₂ interconversion as a measure of extracellular carbonic anhydrase activity (eCA). The enhancement factors ranged from 1.0 (no apparent eCA activity) to 6.4, with an overall mean of 2.9. Additional eCA measurements, made using membrane inlet mass spectrometry (MIMS), yielded activities ranging from 2.4 to 6.9 U · [μg chl a]^?1 (mean 4.1). Measurements of short‐term C‐fixation parameters revealed saturation kinetics with respect to external inorganic carbon, with a mean half‐saturation constant for inorganic carbon uptake (K_1/2) of ～380 μM. Comparison of our early springtime results with published data from late‐season Ross Sea assemblages showed that neither HCO₃^? utilization nor eCA activity was significantly correlated to ambient CO₂ levels or phytoplankton taxonomic composition. We did, however, observe a strong negative relationship between surface water pCO₂ and short‐term ¹⁴C‐fixation rates for the early season survey. Direct incubation experiments showed no statistically significant effects of pCO₂ (10 to 80 Pa) on relative HCO₃^? utilization or eCA activity. Our results provide insight into the seasonal regulation of C uptake by Ross Sea phytoplankton across a range of pCO₂ and phytoplankton taxonomic composition.     

Productivity gains do not compensate for reduced calcification under near‐future ocean acidification in the photosynthetic benthic foraminifer species Marginopora vertebralis

Changes in the seawater carbonate chemistry (ocean acidification) from increasing atmospheric carbon dioxide (CO₂) concentrations negatively affect many marine calcifying organisms, but may benefit primary producers under dissolved inorganic carbon (DIC) limitation. To improve predictions of the ecological effects of ocean acidification, the net gains and losses between the processes of photosynthesis and calcification need to be studied jointly on physiological and population levels. We studied productivity, respiration, and abundances of the symbiont‐bearing foraminifer species Marginopora vertebralis on natural CO₂ seeps in Papua New Guinea and conducted additional studies on production and calcification on the Great Barrier Reef (GBR) using artificially enhanced pCO₂. Net oxygen production increased up to 90% with increasing pCO₂; temperature, light, and pH together explaining 61% of the variance in production. Production increased with increasing light and increasing pCO₂ and declined at higher temperatures. Respiration was also significantly elevated (~25%), whereas calcification was reduced (16–39%) at low pH/high pCO₂ compared to present‐day conditions. In the field, M. vertebralis was absent at three CO₂ seep sites at pH_Total levels below ~7.9 (pCO₂ ~700 μatm), but it was found in densities of over 1000 m^?2 at all three control sites. The study showed that endosymbiotic algae in foraminifera benefit from increased DIC availability and may be naturally carbon limited. The observed reduction in calcification may have been caused either by increased energy demands for proton pumping (measured as elevated rates of respiration) or by stronger competition for DIC from the more productive symbionts. The net outcome of these two competing processes is that M. vertebralis cannot maintain populations under pCO₂ exceeding 700 μatm, thus are likely to be extinct in the next century.     

Iron Limitation Modulates Ocean Acidification Effects on Southern Ocean Phytoplankton Communities

The potential interactive effects of iron (Fe) limitation and Ocean Acidification in the Southern Ocean (SO) are largely unknown. Here we present results of a long-term incubation experiment investigating the combined effects of CO₂ and Fe availability on natural phytoplankton assemblages from the Weddell Sea, Antarctica. Active Chl a fluorescence measurements revealed that we successfully cultured phytoplankton under both Fe-depleted and Fe-enriched conditions. Fe treatments had significant effects on photosynthetic efficiency (F_v/F_m; 0.3 for Fe-depleted and 0.5 for Fe-enriched conditions), non-photochemical quenching (NPQ), and relative electron transport rates (rETR). pCO₂ treatments significantly affected NPQ and rETR, but had no effect on F_v/F_m. Under Fe limitation, increased pCO₂ had no influence on C fixation whereas under Fe enrichment, primary production increased with increasing pCO₂ levels. These CO₂-dependent changes in productivity under Fe-enriched conditions were accompanied by a pronounced taxonomic shift from weakly to heavily silicified diatoms (i.e. from Pseudo-nitzschia sp. to Fragilariopsis sp.). Under Fe-depleted conditions, this functional shift was absent and thinly silicified species dominated all pCO₂ treatments (Pseudo-nitzschia sp. and Synedropsis sp. for low and high pCO₂, respectively). Our results suggest that Ocean Acidification could increase primary productivity and the abundance of heavily silicified, fast sinking diatoms in Fe-enriched areas, both potentially leading to a stimulation of the biological pump. Over much of the SO, however, Fe limitation could restrict this possible CO₂ fertilization effect.     

Limnology and trophic status of glacial lakes in the tropical Andes (Cajas National Park,Ecuador)

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EFFECT OF CARBONIC ANHYDRASE INHIBITORS ON THE INORGANIC CARBON UPTAKE BY PHYTOPLANKTON NATURAL ASSEMBLAGES1

The role of carbonic anhydrase (CA) in inorganic carbon acquisition (dissolved inorganic carbon, DIC) was examined in Alboran Sea phytoplankton assemblages. The study area was characterized by a relatively high variability in nutrient concentration and in abundance and taxonomic composition of phytoplankton. Therefore, the relationship between environmental variability and capacity for using HCO₃^? via external CA (eCA) was examined. Acetazolamide (AZ, an inhibitor of eCA) inhibited the primary productivity (PP) in 50% of the samples, with inhibition percentages ranging from 13% to 60%. The AZ effect was more prominent in the samples that exhibited PP >1 mg C · m^?3 · h^?1, indicating that the contribution of eCA to the DIC photosynthetic flux was irrelevant at low PP. The inhibition of primary productivity by AZ was significantly correlated to the abundance of diatoms. However, there was no a relationship between AZ effect and CO₂ partial pressure (pCO₂) or nutrient concentration, indicating that the variability in the PP percentage supported by eCA was mainly due to differences in taxonomic composition of the phytoplankton assemblages. Ethoxyzolamide (EZ, an inhibitor of both external and internal CA) affected 13 of 14 analyzed samples, with PP inhibition percentages varying from 50% to 95%. The effects of AZ and EZ were partially reversed by doubling DIC concentration. These results imply that CA activity (external and/or internal) was involved in inorganic carbon acquisition in most the samples. However, EZ effect was not correlated with pCO₂ or taxonomic composition of the phytoplankton.     

Intra-annual variation of phytoplankton community responses to factorial N,P, and CO2 enrichment in a temperate mesotrophic lake

1. Across primary producer communities in different lakes, nitrogen (N) and phosphorus (P) can exhibit many different patterns of limitation. Here, we look at the intra-annual variability of these patterns in a single lake. Furthermore, we investigate whether a third resource, carbon dioxide (CO₂) can have significant effects on phytoplankton biomass and community composition.
2. We performed five in situ lacustrine mesocosm experiments at different times of the year. In each experiment, we had a factorial design with two levels of N, P and CO₂ enrichment (no enrichment or double lake concentrations for N and P and atmospheric (400 ppm) and c. 1,000 ppm for CO₂) resulting in a total of eight treatments. Mesocosms of c. 1,600 L were suspended in a temperate, mesotrophic lake (Lac Hertel, Canada). Each experiment lasted 2 weeks and chlorophyll a biomass, coarse chemotaxonomic community composition (measured using fluorometry), and several environmental variables were recorded at a minimum of four time points.
3. We found that the limiting, synergistic, and community composition effects of N and P varied between experiments. TN:TP ratios explained, in part, some of this variability, along with insolation and water temperature.
4. Despite relatively high levels of CO₂ in the control mesocosms, we found a constant synergistic effect of CO₂ with N. In combination with the synergistic effect of P with N found in some experiments, this provides support for CO₂ as one of the multiple limiting resources in nutrient-enriched systems. This finding could have implications for eutrophic lakes exposed to increasing concentrations of CO₂.
5. We also found that the effects of CO₂ on community composition varied intra-annually. Thus, we conclude that generalised predictions about the effect of CO₂ on community composition at a coarse chemotaxonomic scale are unlikely to hold, but predictions specific to season and system are likely to be reliable.

     

Links between Terrestrial Primary Production and Bacterial Production and Respiration in Lakes in a Climate Gradient in Subarctic Sweden

We compared terrestrial net primary production (NPP) and terrestrial export of dissolved organic carbon (DOC) with lake water heterotrophic bacterial activity in 12 headwater lake catchments along an altitude gradient in subarctic Sweden. Modelled NPP declined strongly with altitude and annual air temperature decreases along the altitude gradient (6°C between the warmest and the coldest catchment). Estimated terrestrial DOC export to the lakes was closely correlated to NPP. Heterotrophic bacterial production (BP) and respiration (BR) were mainly based on terrestrial organic carbon and strongly correlated with the terrestrial DOC export. Excess respiration over PP of the pelagic system was similar to net emission of CO₂ in the lakes. BR and CO₂ emission made up considerably higher shares of the terrestrial DOC input in warm lakes than in cold lakes, implying that respiration and the degree of net heterotrophy in the lakes were dependant not only on terrestrial export of DOC, but also on characteristics in the lakes which changed along the gradient and affected the bacterial metabolization of allochthonous DOC. The study showed close links between terrestrial primary production, terrestrial DOC export and bacterial activity in lakes and how these relationships were dependant on air temperature. Increases in air temperature in high latitude unproductive systems might have considerable consequences for lake water productivity and release of CO₂ to the atmosphere, which are ultimately determined by terrestrial primary production.     

CO2 concentrations in perennially ice-covered lakes of Taylor Valley, Antarctica

Lakes in Taylor Valley, southern Victoria Land,Antarctica, are unusual in that they areperennially covered by a 3–5 m thick ice layer.Previous work on gas concentrations in theselakes has shown that the surface waters aresupersaturated with respect to O₂,N₂O, as well as the noble gases. Our datashow that the dissolved CO₂ (CO_2(aq))concentrations, calculated from pH andCO₂, can be highly undersaturatedat shallow depths of the lakes. CO₂partial pressure values (pCO₂) are as lowas 10^–4.3 atm and 10^–4.2 atm in theeast and west lobes of Lake Bonney,respectively, and 10^–3.8 atm in LakeHoare. CO_2(aq) depletion occurred only inthe uppermost part of the water column, inassociation with elevated primary productivity(PPR). The upward diffusion of CO_2(aq)from the aphotic zone, and the annual input ofCO₂ via glacial meltwater can notreplenish the amount of CO_2(aq) annuallylost to primary productivity in the uppermostmeters of the water column. Calcification is alimited source of CO_2(aq), since the lakesare undersaturated with respect to calcitethrough portions of the austral summer.Preliminary respiration rates have been used toobtain an annual inorganic carbon balance.Further down in the water column, at the sitesof the deep-water maximum in primary production(PPR_max), which in Lakes Bonney andFryxell is associated with nutrient gradients,CO_2(aq) is not undersaturated. A largeupward flux from CO₂-supersaturatedaphotic waters provides a surplus ofCO_2(aq) at the PPR_max. Lake Fryxell,unlike the other lakes, is supersaturated withCO_2(aq) throughout the entire water column.     

In situ measurements confirm the seasonal dominance of benthic algae over phytoplankton in nearshore primary production of a large lake

1. Despite the recognition of its importance, benthic primary production is seldom reported, especially for large lakes. We measured in situ benthic net primary production by monitoring flux in dissolved inorganic carbon (DIC) concentration in benthic incubation chambers, based on continuous measurements of CO_2(aq) flux, alkalinity, and the temperature‐dependent dissociation constants of carbonic acid (K₁ and K₂). This methodology has the advantages of monitoring net primary production directly as change in carbon, maintaining continuous water recirculation, and having sufficient precision to detect change in DIC over short (i.e. 15 min) incubations, even in alkaline waters. 2. Benthic primary production on Cladophora‐dominated rocky substrata in western Lake Ontario was measured biweekly. Maximum biomass‐specific net photosynthetic rates were highest in the spring (2.39 mgC g Dry Mass^?1 h^?1), decreased to negative rates by early summer (?0.76 mgC g DM^?1 h^?1), and exhibited a regrowth in late summer (1.98 mgC g DM^?1 h^?1). 3. A Cladophora growth model (CGM), previously validated to predict Cladophora biomass accrual in Lake Ontario, successfully simulated the seasonality and magnitude of biomass‐specific primary production during the first cohort of Cladophora growth. Averaged over this growing season (May–Aug), mean areal net benthic production at the estimated depth of peak biomass (2 m) was 405 mg C m^?2 d^?1. 4. We measured planktonic primary production in proximity to the benthic study and constructed a depth‐resolved model of planktonic production. Using the CGM, benthic primary production was compared with planktonic primary production for the period May–Aug. Net benthic production from the shoreline to the 12 m contour (1–2 km offshore) equalled planktonic production. Closer to shore, benthic primary production exceeded planktonic primary production. Failure to account for benthic primary production, at least during abundant Cladophora growth, will lead to large underestimates in carbon and nutrient flows in the nearshore zone of this Great Lake.     

Associations between water chemistry and fish community composition: a comparison between isolated and connected lakes in northern Sweden

1. The correlation between water chemistry, physical variables and fish community composition was examined in 40 small (≤30 ha) coastal lakes in northern Sweden. Twenty of the 40 lakes were isolated from other water bodies and 20 were connected to the Baltic Sea. Lakes were fished in summer, using three different methods. Water chemistry was sampled in late winter prior to ice‐out and pH was measured additionally in summer. 2. Our central question was whether water chemistry plays a greater role in the composition of fish communities in isolated lakes than in connected lakes, as isolated lakes cannot be recolonised once a species has become extinct. 3. Results indicate that winter anoxia affects community composition only in isolated lakes, whereas acidity is of importance in both connected and isolated lakes. Methane (indicating anoxia), was significantly correlated with variation in fish community composition in isolated lakes, and a group of variables that indicate anoxia (CH₄, pCO₂, inorganic carbon and dissolved oxygen) explained 24–34% of the variation. pH alone explained 12% of the variation in community composition for connected lakes and a group of variables indicating acidity (summer and winter pH and ANC) explained 10–20% of the variation in isolated lakes. Lake area was the most important physical variable, being significantly correlated with the variation in fish community composition in connected lakes. 4. In isolated lakes, the presence of pike (Esox lucius), perch (Perca fluviatilis) and roach (Rutilus rutilus) was associated with low CH₄. The occurrence of crucian carp (Carassius carassius) and roach was positively correlated with pH, and the crucian carp was apparently also affected by predation by pike and perch. In connected lakes the effect of anoxia was low, probably due to the possibility of recolonisation and pockets of oxygenated water, allowing pike and perch to persist and thereby limiting the distribution of crucian carp.