V. Straškrabová, C. Callieri and J. Fott (Guest Editors) Pelagic food web in mountain lakes. MOuntain LAkes Research Program J. Limnol., 58(2): 144-151, 1999
Carbon partitioning in the food web of a high mountain lake: from bacteria to zooplankton Cristiana CALLIERI*, Alessandra PUGNETTI and Marina MANCA C.N.R. Istituto Italiano di Idrobiologia, 28922 Verbania-Pallanza, Italy *e-mail correspondig author:
[email protected]
ABSTRACT The organisms of the microbial loop in Lake Paione Superiore (LPS), a high mountain lake in the Italian Alpine region, were studied together with phytoplankton and zooplankton for three successive years. The biomass of bacteria, HNF (heterotrophic nanoflagellates), ciliates and phytoplankton, as mean carbon concentration in the three years, was 30 and 37 µg C l-1 near the surface (SUR) and the bottom (BOT) respectively. Under the ice-cover the mean biomass carbon decreased especially at the BOT, whereas at SUR the decrease was less evident due to the maintenance of higher phytoplankton biomass (mixotrophic flagellates). In LPS ~50% of the carbon was confined in bacteria, 20% in protozoa and 30% in phytoplankton. The ratio Autotrophs/Heterotrophs was lower than 1 (mean: 0,97 at SUR and 0,58 at BOT) thus indicating a system with a predominance of the heterotrophs. This might be the result of light inhibition of algal growth coupled to a production of dissolved carbon, utilized by bacteria. During late summer the peak of Daphnia longispina, the main component of the zooplankton of LPS, increased the carbon content in the lake to a total of 158 and 300 µg C l-1 in 1997 and 1998 respectively. At the late summer peaks, zooplankton represented from 78 to 89% of the total carbon of the pelagic communities. Furthermore, the presence of Daphnia could be responsible for a decrease in the biomass carbon of a variety of organisms (algae, protozoa and bacteria). It may be possible that this is an instance of zooplankton grazing on algae, protozoa and also bacteria, as Daphnia has very broad niches and may eat pico-, nanoplankton and small ciliates. In the oligotrophic LPS, a diet which also includes protozoa could give Daphnia a further chance of survival, as ciliates are an important source of fatty acids and sterols. Key words: microbial loop, high mountain lake, phytoplankton, zooplankton
1. INTRODUCTION Since the importance of the microbial loop in ecosystem functioning was recognized (Pomeroy 1974; Williams 1981; Sieburth & Davis 1982; Azam et al. 1983), limnologists have devoted more and more effort to studying this part of the food web. Researches focussed on standing stock quantification (Amblard et al. 1995; Hadas & Berman 1998) and on carbon flux measurements in the loop (Weisse 1990; Nagata 1988, Nagata et al. 1996; Amblard et al. 1994; Šimek et al. 1995). Micro and mesocosm experiments were performed (Jürgens 1994; Jürgens et al. 1997) to investigate the food linkage among metazoans, protozoans and the bacterial population. Different studies emphasized the interactions among microorganisms and organisms in the microbial and classical food web in lakes (see review by Jürgens 1994; Pace et al. 1990; Carrick et al. 1991; Mathes & Arndt 1995; Šimek et al. 1997). Nevertheless, information is limited on carbon partitioning of natural assemblages of all the components of the microbial loop and zooplankton simultaneously (Porter 1996). Stockner & Porter (1988) outlined the different efficiency in carbon transfer in two food chains, one dominated by Daphnia and the others by picoplankton and microheterotrophs: the longer the chain the higher the losses. The microbial loop may be a sink for energy
flow, as much carbon is respired at different trophic levels so that the chain tends to support a lower standing stock of fish. But what happens if in the lake there are no fish at all, the water is acid, cold, ice-covered for five-six months and with a clear, light-unprotected water column? This question prompted us to begin a study of the entire food web, from bacteria to zooplankton, of a simplified high mountain lake ecosystem which was slightly acid in 1991-1993 (Mosello et al. 1993), to quantify the carbon in the different compartments of the food web. The study constitutes the part of the EU MOLAR Project involved in Pelagic Food Web Research. Specifically, this was a three-year study on the pelagic community composition of the high mountain lake: Lake Paione Superiore (LPS) in the Italian Central Alps above 2000 m (Mosello et al. 1993, 1994). The absence of fish, the simplification of the other compartments with relatively few species and the almost exclusive presence of Daphnia in the summer zooplankton population (Cammarano & Manca 1997) made this small lake unique for studying the microbial loop and its relationship with Daphnia and phytoplankton. In this paper we will mainly discuss the carbon partitioning in the food web during a three-year study, comparing the carbon content of the different autotrophic and heterotrophic compartments in the ice-free and ice-cover periods. In a more detailed paper published in
Carbon partitioning in food web
this issue (Callieri & Bertoni 1999) the study of the dynamics of the components of the microbial loop will be described also in relation to changes in pH and the light climate. Another paper will show the results of the activity measurements of autotrophic production, bacterial production and protozoan grazing (Callieri & Bertoni, in preparation). 2. MATERIALS AND METHODS Field samplings were performed monthly from July 1996 to October 1998 at the point of maximum depth (11.7 m) of Lake Paione Superiore (LPS), a small glacial cirque lake at an altitude of 2269 m in the Bognanco Valley (Pennine Alps, Italy) (Bianchini 1952). Its geographic and morphometric features, together with the hydrochemical variables measured in 1991-1994, were reported in synthesis by Cammarano & Manca (1997). More recent data on water chemistry can be found in The MOLAR Water Chemistry Group (1999, this issue). A light extinction profile during summer and winter is presented by Callieri & Bertoni (in prep.). The water column is well irradiated, with 12% of surface irradiance present at the bottom. The measurements of underwater light, as PAR (µE m-2 s-1), were performed with a LI-COR 250, and the extinction coefficient (k) was 0.22-0.32 m-1 in summer. The k values measured in 1991-93 were 0.1-0.2 m-1 (Bettinetti 1994; Pugnetti & Bettinetti 1995). The temperature in August is around 14 °C and a stratification appeared in August 1997 and July 1998. During the ice-free period, we sampled picoplankton, protozoa, phytoplankton and zooplankton at two depths, namely, one meter under the surface in summer (SUR) and one meter above the bottom (BOT). Except for zooplankton, on which information was already available, we also collected samples under the ice-cover (one meter under the ice-water interface and one meter above the bottom). Zooplankton net integrated samples were also collected on each date in replicates, to obtain the carbon biomass directly from dry weight and carbon content (for details see Manca & Comoli 1999, this issue). To fix and count bacteria or HPP (heterotrophic picoplankton), APP (autotrophic picoplankton), HNF (heterotrophic nanoflagellate), phytoplankton and ciliates, the protocols described by Straškrabová et al. (1999, this issue) were followed, to obtain results comparable with the other lakes included in the MOLAR project. One exception was the dyeing of bacteria, which was done using Acridine Orange instead of DAPI, and the use of Anopore filters instead of Nuclepore membranes of the same porosity (0.2 µm pore size diameter) (Straškrabová et al. 1999, this issue). All the counting and biovolume measurement data were converted to carbon using the conversion factors and the equations reported in the protocols (Straškrabová et al. 1999, this issue). The statistical analyses performed were ANOVA tests.
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3. RESULTS In figure 1 the biomass temporal variations of HPP, APP, HNF, ciliates, phytoplankton and zooplankton are shown at SUR and BOT depths respectively. The depth of the BOT changed as the lake level fluctuated: in winter it was 7-8 m and in summer 9-10 m. These depths guaranteed sampling without contamination due to sediment resuspension. APP, both eukaryots and cyanobacteria, were almost absent, reaching a maximum value of 0.1 µg C l-1. On these rare occasions they were small eukaryotic algae, as picocyanobacteria were absent. Stockner & Shortreed (1991) have reported a situation of complete absence of pico-blues and dominance of eukaryotic picoplankton in low pH, humic Danish lakes. Both the acid conditions of LPS waters and the possibility of PAR and UV inhibition at 2200 m could be evoked as the possible causes of the absence of picocyanobacteria. HPP (bacteria) constituted around 50% of the carbon of the microbial food web (microbial loop plus phytoplankton) and their seasonal dynamics were quite similar at the two depths, with the sole exception of early summer 1998, when an isolated peak of 56 µgC l-1 was measured, due to the presence of large rods and long filaments at the BOT. Under the ice the bacterial carbon concentration was around 5 µg C l-1, which nevertheless represents 40% of the organic carbon in the microbial food web. During early summer the bacterial carbon concentration, as a mean, was 36 µg C l-1. HNF showed a density range from 4 104 l-1 to 4 106 -1 l and were composed of a predominance of
