SCIENTIFIC CORRESPONDENCE trite extracts (AD., unpublished results). m1tiat1on of haem polymerization might onset of stratification was obtained by Thus, an appropriate pH and an envir­ occur1• However, we stress that there is no combining three measurements from onment allowing initiation of polymer­ evidence yet for a haem polymerase early June profiles: the depth of the ization, possibly involving lipid, can enzyme as such, and that there is no need maximum temperature gradient, the rate account for the formation of parasite to invoke one to explain the currently of deepening of the 9° isotherm and the haemozoin. available data. difference in depth of the 9° and 10° Bendrat et al. leave open the possibility Robert G. Ridley, Arnulf Dorn isotherms. These measurements were that a haem polymerase enzyme may yet Hugues Matile, Manfred Kansy combined to form the index by subtracting be found to exist in the parasite. We have Pharma Division, Preclinical Research, the mean, dividing by the standard previously postulated that protein(s) may F. Hoffmann-La Roche, deviation, and then subtracting the last serve as a structural focus about which the CH-4002 Basel, Switzerland measurement (which decreases with intensity of stratification) from the sum of the first two (which increase with intensity UK lake plankton and the Gulf Stream of stratification). The relationship between the biomass SIR - Recent studies suggest that plank­ North Basin of Lake Windermere and the of zooplankton and this empirical index of ton succession in temperate lakes is strong­ latitude of the Gulf Stream (see a in the fig­ stratification (b in the figure) shows that ly influenced by year-to-year variations in ure). An important factor influencing the when the lake becomes thermally strati­ the weather1• Here we show that the inter­ seasonal dynamics of zooplankton in Lake fied in early summer, the biomass of zoo­ annual vanat1ons in the average Windermere is the timing and intensity of plankton is much lower than when the summer biomass of zooplankton in Lake thermal stratification4• When the lake lake becomes stratified later in the season. Windermere between 1966 and 1991 were becomes stratified in early summer, the The year-to-year variations in thermal closely correlated with north-south dis­ growth of edible algae occurs before the stratification of Lake Windermere are placements of the Gulf Stream in the west­ zooplankton are able to lay their full com- in turn closely correlated with the ern Atlantic. The mediating north-south movements of the Gulf factor seems to be subtle a 2~------~ Esthwaite Water, a much European Shelf 2, Lake Win­ 'al) smaller and shallower lake in the same -2 +--~--~--~-~ - ---'- .2 dermere seems to be a par­ 65 70 75 80 85 90 area, is also shown in part c in the figure. ticularly striking integrator Year We believe that the results reported of climatic events. here and in the marine plankton demon­ b C 4------~ Lake Windermere is the X strate a hitherto unsuspected link between largest lake in the English §'f -0.1 • • Q) weather patterns around the United King­ :!.: E • ~ 2 Lake District (54° 22'N, 2° • • C dom and oceanic events at the other side ~~ •• 0 C. Ambleside, Cumbria LA22 OLP, UK waite Water. Calcul ations for Esthwaite Water used the 11" and Gulf Stream latitude was 12" C isotherms. Values of rare 0.65 and 0.56, respectively, for A. H. Taylor· constructed from these Lake Windermere and Esthwaite Water. Each rvalue has P<0.01 Plymouth Marine Laboratory, charts using principal com­ after allowing for serial correlation (as in ref. 3). Prospect Place, ponents analysis2• West Hoe, Plymouth PL1 30H, UK Correlations between marine plankton plement of eggs, whereas if the lake * To whom correspondence should be addressed. abundance and the position of the Gulf becomes stratified later in the summer, the 1 . Reynolds, C. S. in Organisation of Communities Past and Stream seem to be related to local growth of edible algae is delayed and there Present (eds Gee, J. H. R. & Giller, P. S.) 297- 325 (Blackwell , Oxford, 198 7). variations in the intensity of thermal is a better match between the zooplankton 2. Taylor, A. H .. Colebrook, J.M., Stephens, J. A. & Baker, stratification2.3, so similar effects might be and their food. N. G. J. mar. biol. Ass. U.K. 72, 9 19-921 (1992). 3. Taylor, A. H. Int. J. Climatol. (in the press). anticipated in inland waters on the These effects of early summer stratifi­ 4 . George, D. G. & Harris, G. P. Nature 316, 536-539 European Continental Shelf. There is a cation can be illustrated by analysing the (1985). 5. Willis, A. J. et al. Oikos 73, 408- 410 (1995). strong negative correlation between the thermal characteristics of the lake at the 6 . George, D. G. & White, N. J. J. Plankt. Res. 7, 411- 414 summer biomass of zooplankton in the beginning of June. A general index of (1985). NATURE · VOL 378 · 9 NOVEMBER 1995 139