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

Abstract

1. Five oligotrophic clear-water lakes on the Faroe Islands were studied during August 2000. Algal and bacterial production rates, community respiration, and CO 2 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 CO 2 (830-2140 latm) 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.

Organic matter degradation through respiration in littoral and pelagial including profundal zones of an oligotrophic lake assessed by electron transport system activity

“…Our results are in agreement with several studies that have suggested that respiration exceeds photosynthesis in the epilimnion of oligotrophic lakes (del Giorgio & Peters, 1994;Duarte & Agustí, 1998;Pålsson et al, 2005;Simčič, 2005;Karlsson et al, 2007). del Giorgio & Peters (1994) reported that plankton community respiration was 2-to 8-fold greater than phytoplankton photosynthesis in oligotrophic and mesotrophic lakes during the growing season, but that this imbalance decreased in the more productive lakes.…”

“…Primary production by phytoplankton, macrophytes, and benthic algae forms the autochthonous basis of the food web. Terrestrial input of particulate and dissolved organic carbon is a potential allochthonous basis of the food web (Cole et al, 2000;Pålsson et al, 2005). Ecosystems in which photosynthesis exceeds total respiration (P [ R) are net autotrophic-they are net sinks for CO 2 and net producers of O 2 and organic matter.…”

Microbial planktonic communities of freshwater environments from Tierra del Fuego: dominant trophic strategies in lakes with contrasting features

“…On the one hand, mixotrophy tends to be a suitable strategy in environments with low nutrients and good light climate (i.e. oligotrophic clear lakes), thus mixotrophs tend to dominate over strictly autotrophic organisms due to their capability to incorporate nutrients by prey ingestion (Medina-Sa ´nchez et al, 2004;Pa ˚lsson et al, 2005). On the other hand, in environments with humic coloured waters characterized by relatively low Kd values and high a 320 absorbances, the mixotrophic flagellates that are able to turn from autotrophy to heterotrophy to supplement their carbon budget under light limitation may thrive.…”

The relative importance of heterotrophic bacteria to pelagic ecosystem dynamics varies with reservoir trophic state

“…The relationship of BPr : PPr with Chl a was generally more variable (r 2 5 0.42), especially in our more oligotrophic reservoirs, than would be expected from the strong relationships of BPr and PPr with Chl a (r 2 5 0.84 and 0.89, respectively). In their study of oligotrophic lakes on the Faroe Islands, Palsson et al (2005) found that areal BPr : PPr also ranged widely (0.21-0.95). The higher variance of BPr : PPr in more oligotrophic systems could be caused by bottom-up control of BPr, whereas top-down control may be more common in eutrophic systems.…”