Limnol. Oceanogr., 44(2), 1999, 436–439

Recent decades have seen a large increase in surface scums (blooms) of cyanophytes (blue-green algae and blue-green bacteria) in inland waters. These are potentially toxic to mammals, including humans, and have caused considerable public concern in Europe, Australasia, and North America. They are often associated with eutrophication, and much has been invested in their control. Not all blooms, however, are necessarily the results of human interference with lakes. Scattered paleolimnological evidence indicates that some blooms may be associated with pristine conditions, though this message has largely been ignored. Evidence is given here of a long history of blooms in Whitemere, U.K., from extraction and identification of specific carotenoids from dated sediment cores. Whitemere is representative of a large group of lakes in the West Midlands of the U.K. and is likely to be representative also of those similar postglacial kettle hole lakes in North America and Eurasia, which are groundwater fed with long retention times and thermally stratified. Bluegreen blooms may thus be a normal feature of such lakes and not necessarily a pathology to be controlled. Blue-green algal blooms are now very common. Many lowland lakes in Europe have carried, in some summers, warning signs against the dangers of toxicity (Codd 1984; Codd and Beattie 1991), and regulatory authorities are widely concerned about reducing the incidence of blooms and improving amenity (National Rivers Authority 1990). Conservation values have been reduced (Carvalho and Moss 1995). Even more serious are concerns about public water supply, for blooms are costly in water treatment (Hayes and Greene 1984). There has inevitably developed a culture in which blooms are universally vilified and associated with human alteration of catchments and discharge of effluents. Concern is often justified, but blooms are not always recent phenomena. Paleolimnological evidence sometimes reveals anthropogenic bloom incidence long before the modern period (Hutchinson et al. 1970; van Geel et al. 1994). There also exists, however, a small amount of paleolimnological literature hinting at an even more ancient incidence of blooms, prior to human settlement. Often, these are associated with increased stabilization of the water column in warming climates. Zullig (1989) records an increase in cyanophyte pigments in the early Atlantic period, persisting to the mid-Holocene in the Soppensee and (1986) in the Preboreal and Boreal in the Lobsigensee. Fritz (1989) notes an expansion of Oscillatoria about 6000 B.P. in a small East Anglian lake. In Alberta, Lake Wabamum and other lakes have had prolonged incidence of large blue-green algal populations, waxing and waning with climate change and fire history in the surrounding prairie, but persisting as a constant feature (Schweger and Hickman 1989; Hickman and Schweger 1991). Compared with the number of lakes in the northern hemisphere, systematic paleolimnological investigations have been few, and those capable of revealing changes in blue-green algae, as opposed to diatoms, have been even fewer. If Lake Wabamum had blooms as normal features, perhaps many other lakes have had similar histories. The North-West Midland meres, U.K. (Reynolds 1979), are well known for their cyanophyte blooms. The meres were formed around 10,000–14,000 yr ago on the glacial drift plain of Shropshire and Cheshire, by moraine damming, kettle hole formation, and the collection of meltwater in hollows, sometimes created by saline subsidence, in the hummocky landscape. Early bloom formation in them is reflected in a local expression—‘‘the breaking of the meres,’’ which is taken from a term formerly used in brewing to describe the rising of the wort (yeast) to the surface during fermentation (Phillips 1884). There are also literary references to such ‘‘breaking’’ in the seventeenth century (Webb 1928). Blooms in the historic period, however, might easily be attributed to local eutrophication from village sewage or farms, but they suggest the possibility of a much earlier origin. Were this the case, and because the meres are in no way unusual among thousands of small, stratified, groundwater-fed lakes on the glacial plains of the northern hemisphere, the commonly believed paradigm of blooms as inevitable pathological features of lakes might need to be revisited and approaches to lake restoration adjusted. Whitemere, in North Shropshire (28529W, 528539N; National Grid Reference SJ 415330), is a typical kettle hole lake with annual cyanophyte blooms, usually dominated by Anabaena circinalis and Microcystis aeruginosa (Kilinc 1995). It has an area of 25.5 ha, a maximum depth of 17 m, and a monomictic thermal regime. Conductivity is high (300–350 ms cm21), alkalinity is moderate (1.7 6 0.1 meq liter21, and pH ranges from 6.7 to 9.8. Soluble reactive and total phosphorus concentrations span 0.5 to .1.0 mg liter21, which is not atypical of stratified meres in the region (Reynolds 1971; Moss et al. 1994, 1997). There are no point sources of eutrophication and no surface inflows. The mere is fed by groundwater, which has low phosphorus concentrations (Kilinc 1995) and is groundwater drained. Winter inorganic nitrogen concentrations range from 0.5 to 1.5 mg N liter21, and inorganic nitrogen is depleted in summer. Bioassays (Hameed et al. in press) suggest that algal yield is then limited by nitrogen. Sediment cores were taken with a Mackereth corer (Mackereth 1958) from a flat area of lake bottom under 7 m of water to avoid possible complications of sediment slumping in the precipitous hole that contains the deepest water. Cores reached the junction of the lake sediment and the glacial clay. Radiocarbon-dated organic lake sediment from close to the bottom had an age of 6540 6 80 B.P. (SRR-5687), and a further radiocarbon date of 860 6 70 B.P. (SRR-5686) was obtained at 130 cm. These dates are unlikely to be affected by carbonate effects, for carbonate comprised at most 3% of the dry sediment in the lower part of the core and was lower