Light limitation of phytoplankton development in an oligotrophic lake ‐ Loch Ness, Scotland

Abstract

S U M M A R Y1. The underwater light climate in Loch Ness is described in terms of mixing depth (Z,) and depth of the euphotic zone (Zeu). During periods of complete mixing, Z, equates with the mean depth of the loch (132 m), but even during summer stratification the morphometry of the loch and the strong prevailing winds produce a deep thermocline and an epilimnetic mixed layer of about 30 m or greater. Hence, throughout the year the quotient ZJZ, is exceptionally high and the underwater light climate particularly unfavourable for phytoplankton production and growth. 2. Phytoplankton biomass expressed as chlorophyll a is very low in Loch Ness, with a late summer maximum of less than 1.5 mg chlorophyll a m-3 in the upper 30 m of the water column. This low biomass and the resulting very low photosynthetic carbon fixation within the water column are evidence that a severe restraint is imposed on the rate at which phytoplankton can grow in the loch. 3. The chlorophyll a content per unit of phytoplankton biovolume and the maximum, light-saturated specific rate of photosynthesis are both parameters which might be influenced by the light climate under which the phytoplankton have grown. However, values obtained from Loch Ness for both chlorophyll a content (mean 0.0045 mg mm-3) and maximum photosynthetic rate (14 mg C mg Chla-' h-*) are within the range reported from other lakes. 4. Laboratory bioassays with the natural phytoplankton community from Loch Ness on two occasions in late summer when the light climate in the loch is at its most favourable, suggest that even then limitation of phytoplankton growth is finely balanced between light and phosphorus limitation. Hence, for most of the year, when the light climate is less favourable, phytoplankton growth will be light limited. 5. Quotients relating mean annual algal biomass as chlorophyll a (c. 0.5 mg Chla m") and the probable annual specific areal loading of total phosphorus (0.4-1.7 g TP m-2 yr-l) suggest that the efficiency with which phytoplankton is produced in Loch Ness per unit of TP loading is extremely low when compared with values from other Scottish lochs for which such an index has been calculated. This apparent inefficiency can be attributed to suppression of photosynthetic productivity in the water column due to the unfavourable underwater light climate. 6. These several independent sources of evidence lead to the conclusion that phytoplankton development in Loch Ness is constrained by light rather than by nutrients. Loch Ness thus appears to provide an exception to the generally accepted paradigm that phytoplankton development in lakes of an oligotrophic character is constrained by nutrient availability. 0 1996 Blackwell Science Ltd 533 534 R. 1. Jones et al. II. Nutrient depletion and the spring maximum. Journal Lund J.W.G. (1954) The seasonal cycle of the plankton diatom Melosira italica (Eh.) Ktitz. Subsp. subarctica 0.

Control of bacterioplankton growth and abundance in deep, oligotrophic Loch Ness (Scotland)

“…At the start of the study, during the winter of 1996, the bacterial growth rate was very low, but the highest rates recorded during the study actually occurred in the following winter of 1997. Low values actually occurred during the late summer when the phytoplankton biomass is maximal in Loch Ness (Jones et al 1996). This is in marked contrast to other lakes (e.g.…”

“…The deep mixing and poor light penetration into the moderately humlc water mean that the underwater light climate experienced by the phytoplankton in the loch is exceptionally unfavourable, and the maximum daily rates of area1 phytoplankton photosynthesis achleved in Loch Ness are only around 100 mg C m-' d-' (Jones et al 1996). This very low phytoplankton production in Loch Ness (Jones et al 1996) is clearly insufficient to support the bacterial production (Laybourn-Parry et a1 1994), implying that bacteria in the loch are dependent on the influx of allochthonous DOC from the catchment.…”

“…This very low phytoplankton production in Loch Ness (Jones et al 1996) is clearly insufficient to support the bacterial production (Laybourn-Parry et a1 1994), implying that bacteria in the loch are dependent on the influx of allochthonous DOC from the catchment. In practice, the total organic carbon content of the loch is dominated by detrital carbon of allochthonous origin (Jones et al 1997).…”

“…The samples were returned to the laboratory in an insulated box. The water temperature at the time of sampling was measured with a YSI have shown that bacterioplankton effec- (Jones et al 1996) probe and meter. Oxygen measurements were not taken, but previous studies (Maitland 1981, LaybournParry et al 1994 have shown that the entire water column in Loch Ness is always close to 100% sat.urated with oxygen.…”

“…The bottles were incubated (6 d) in the dark in a water bath set to the temperature of the loch at the time of sampling. A separate sample with no added nutrients was incubated at 16°C approximating the annual maximum temperature attained in Loch Ness (Jones et al 1996). All treatments were performed in triplicate.…”