Journal of Plankton Research Vol.15 no.9 pp.1035-1051, 1993. Temperature and Antarctic plankton community respiration. Carol Robinson and Peter J.leB.
Journal of Plankton Research Vol.15 no.9 pp.1035-1051, 1993
Temperature and Antarctic plankton community respiration Carol Robinson and Peter J.leB.Williams University of Wales: Bangor, School of Ocean Sciences, Menai Bridge, Gwynedd, LL59 5EY, UK Abstract. Antarctic plankton community respiration rates were determined from in vitro changes in dissolved oxygen. Oxygen consumption rates, measured at in situ temperatures between 0 and 6°C, were found to lie in the range 0.3-3.7 ujnol O2 I"1 per 24 h. Water samples were collected between East Falkland Island and South Georgia, South Atlantic Ocean, and incubated shipboard in the dark at up to 36 temperatures between - 2 and 14°C. A respiration rate at each temperature was then determined and used to calculate the temperature coefficient (Q,u) of Antarctic planktonic community respiration from the Arrhenius equation. Fourteen Qm values lay in the range 1-3, with four further values >5. This range of temperature coefficient values for community respiration is comparable to the published range of values for plankton photosynthesis. Frequency distributions of temperature coefficients for the two processes show similar modal Qu$ of 2-3. Thus, this study does not lend support to the hypothesis of a differential response of photosynthesis and community respiration to low temperature.
Introduction
Temperature is an important regulator of plankton metabolic activity (Morita, 1975; Neon and Holm-Hansen, 1982; Jacques, 1983; Li et al., 1984; Tilzer and Dubinsky, 1987; Kottmeier and Sullivan, 1988; White et al., 1991). Particularly in the nutrient-rich conditions of the Antarctic Ocean, temperature is argued to be a major environmental parameter controlling plankton growth (Hanson and Lowery, 1985; Tilzer et al., 1986; Tilzer and Dubinsky, 1987). The growth or decay of a phytoplankton bloom is dependent on the balance of photosynthesis and algal and heterotrophic respiration. Thus, any differential sensitivity of photosynthesis and respiration to temperature will be critical in determining community structure and carbon flow. Pomeroy and Deibel (1986), in a study of microbial growth and respiration in Newfoundland coastal waters, observed a different response of phytoplankton and bacteria to low temperatures. Phytoplankton photosynthesis and bacterial respiration were both suppressed at low temperatures; however, at temperatures between +1 and -1°C, photosynthesis was significant, while rates of bacterial respiration were lower by comparison. Later experiments (Pomeroy et al., 1991; Wiebe et al., 1992) suggest that bacterial growth rate at low temperatures is much more dependent on substrate concentration than it is at higher temperatures. Li and Dickie (1987), working in the Bedford Basin, Nova Scotia, a eutrophic temperate coastal embayment, and Kottmeier and Sullivan (1988), studying sea ice microbial communities isolated from McMurdo Sound, Antarctica, show phytoplankton and bacteria exhibiting similar metabolic and growth responses at low temperatures. Thus, the question of plankton community response to temperature is still debated. The temperature coefficient (C10) of photosynthesis is well documented 1035
C.Robinson and P.J.IeB.Williams
(Raven and Geider, 1988) and there is some measure of consensus. In contrast, Qxo values for plankton community respiration are, at best, sparse. Until recently, plankton community metabolism, either as respiration or growth, has been technically difficult to measure using chemical techniques because of the low rates encountered. The sensitive radiochemical methods (e.g. [3H]thymidine incorporation into DNA and [3H]leucine incorporation into proteins), although providing a reasonable estimate of bacterial growth rate (Riemann and Bell, 1990), have a number of associated interpretative problems ranging from isotope activity and small incubation volume effects (Li and Dickie, 1987) to the decision as to which conversion factor to use in the calculations. In the specific case of Antarctic bacteria, a further problem lies in the suggestion that a large proportion of the population may be unable to incorporate exogenous [3H]thymidine into DNA (White et al., 1991). The advent of high-precision analyses for dissolved oxygen (Bryan etal., 1979; Tijssen, 1979; Williams and Jenkinson, 1982), and carbon dioxide (Robinson and Williams, 1991), has meant that a number of these problems can now be circumvented. In the present study, we examine the effect of temperature on Antarctic plankton community respiration, as determined by measured rates of in vitro oxygen consumption. Method The study was performed during a 5 week cruise of the British Antarctic Survey research vessel RRS 'John Biscoe', between East Falkland Island and South Georgia, latitudes 52°50'S to 57°55'S and longitudes 35°20'W to 55°20'W. Sampling station positions are shown in Figure 1. Water samples were collected either with a 30 1 Goflo sampling bottle, or from a pumped seawater supply with an intake at a depth of ~3 m. Two types of incubations were undertaken. (i) Shipboard incubations: water was collected and distributed into calibrated borosilicate glass bottles of nominal volume 150 cm3. These bottles were placed inside opaque plastic bags and incubated for 24 h on board ±0.5°C of in situ temperature. Five profiles, each of 5-8 depths, and five surface samples were analysed in this way. (ii) Temperature gradient incubations: a 20 1 water sample was siphoned into a pre-rinsed polypropylene aspirator and equilibrated with air for 1-4 h at 6°C. The water was then distributed into calibrated borosilicate glass bottles. These were incubated in the dark in an aluminium temperature gradient block for 24 h at temperatures between 3 and 14°C (±0.2°C). A 35.5 x 61 x 10 cm block was machined from a billet of marine-grade aluminium alloy. Twenty-four holes of 6 cm diameter were bored to hold the oxygen bottles, six along the length and four across the width. The block was insulated with 2.5 cm thick expanded polystyrene, the whole being enclosed within a wooden box. Heat-exchange channels were milled into the ends of the block, allowing the connection of a cooler circulator at one end and a heater 1036
Temperature and Antarctic plankton respiration
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circulator at the other. The temperature of the coolant was adjusted to maintain a temperature gradient of ~0.3°C cm"1 along the length of the block. Thermal contact between the bottles and the block was made by water. Incubations at temperatures 14°C, in agreement with the observations of Li and Dickie (1987). Samples were collected from depths between 3 and 75 m, and from stations within and on either side of the Antarctic Convergence; in situ temperatures 1045
C.Robinson and P.J.IeB.Williams
ranged from 1 to 6°C. No correlation between temperature coefficient and depth in the water column or in situ temperature could be found. Similarly, Li et al. (1984) found no difference between the £>10s of the photosynthetic capacity of Arctic and Subarctic phytoplankton, and Packard et al. (1975) showed that the Arrhenius activation energy for respiratory electron transport activity of microplankton was not significantly dependent on habitat temperature between 2 and 20°C. No correlation was found between the Q]0 of plankton community respiration and latitude, within the limited area sampled. Li (1985), in a study of the photosynthetic response to temperature along the latitudinal gradient 16-74°N, observed a decrease in temperature coefficient from low to high latitudes. In contrast, Smith and Platt (1985) found Arrhenius activation energies (£ a ) for ribulose biphosphate carboxylase (RuBPC) activity to be significantly higher for polar than for tropical phytoplankton. In situ small community respiration rates Few respiration data for polar seas exist in the literature. Williams (1984), in a review of plankton community respiration, cites 0-16 mg O2 m~3 day"1 (0-0.5 u,mol O 2 I"1 per 24 h) as the range for the Antarctic, whereas Harrison (1986) gives a mean of 60 mg O2 m" 3 day'' (1.9 nmol O2 I"1 per 24 h) for the Canadian Arctic, calculated from in situ profiles between 0 and 100 m. In the present study, five profiles of respiration were determined. Respiration rates at depths between 0 and 100 m (Table II) lay in the range 16-120 mg O2 m~3 day"1 (0.53.69 ^mol O 2 r ' per 24 h). Community characterization The temperature coefficients calculated here are for small community respiration, i.e. for a combination of bacterial, algal and protozooplankton respiration in the nano-, pico- and micro-size classes. The population sampled is set by the volume of the incubation bottles (23456789
