Eutrophication can initiate sudden ecosystem state change either by slowly pushing lakes toward a catastrophic tipping point beyond which self‐reinforcing mechanisms establish an alternate stable state, or through rapid but persistent changes in external forcing mechanisms. In principle, these processes can be distinguished by determining whether historical changes in focal parameters (phytoplankton) exhibit transient (rising then declining) or continuously‐elevated variability characteristic of alternate stable states or a “paradox of enrichment,” respectively. We tested this hypothesis in the south basin of Lake Winnipeg, Canada, a site with intense blooms of N2‐fixing cyanobacteria since 1990, but for which little is known of earlier limnological conditions, causes of eutrophication, or whether modern conditions represent a alternate stable state. Paleolimnological analysis revealed that the basin was naturally mesotrophic (∼15–20 μg P L−1) with diazotrophic cyanobacteria, productive diatoms, and phosphorus‐rich sediments. Eutrophication accelerated during ca.1900–ca.1990, when sedimentary nitrogen, phosphorus and carbon contents increased 10–50%, δ15N enriched 3–4‰, and concentrations of many fossil pigments increased 300–500%. Nearly 75% of 20th century variability was explained by concomitant increases in production of livestock and crops, but not by climate. After ca.1990, the basin exhibited a rapid threefold increase in akinetes from Aphanizomenon and Anabaena spp. and 50% declines in pigments from chlorophytes and cyanobacteria because of sudden socio‐economic reorganization of agriculture. Phytoplankton variability quantified using Gaussian generalized additive models increased continuously since the onset of agriculture for bloom‐forming taxa, did not decline after state change, and suggested that recovery should not be affected by stable‐state hysteresis.

Biological responses to a formerly fi shless lake in the Cascade Range, Oregon (USA) were assessed through monitoring of recent changes and paleolimnological techniques to assess earlier changes. We used un- published fi sheries reports, sediment cores, and available published and unpublished water quality data to evaluate changes to the lake. Diamond Lake has undergone four periods of fiintroductions in the 20 th century. Rainbow trout (Salmo gairdneri) were annually released from 1910-2006, except for 1949 and 1954. Tui chub (Gila bicolor), an omnivorous cyprinid, were introduced in the late 1930s and the late 1980s, presumably as discarded live bait. Diamond Lake was treated with rotenone in 1954 which successfully eradicated the tui chub. The introductions of trout caused relatively modest changes in water quality and lake biota, whereas the introductions of tui chub caused major increases in cyanobacteria, changes in diatom community composition, reduction in transparency, increases in the proportion of rotifers, major reduction in benthic standing crop, and virtual elimination of amphipods, gas- tropods, and other large-bodied invertebrates. The unintended biomanipulations demonstrate the importance of an omnivorous cyprinid in promoting a series of biological responses throughout the lake food web.

Sediment cores were collected from Upper Klamath Lake in October, 1998 and analyzed for 210Pb, 14C, 15N, N, P, C, Ti, Al, diatoms, Pediastrum, and cyanobacterial akinetes. These results were used to reconstruct changes in water quality in Upper Klamath Lake over the last 150 years. The results showed that there was substantial mixing of the upper 10 cm of sediment, representing the previous 20 to 30 years. However, below that, 210Pb activity declined monotonically, allowing reasonable dating for the period from about 1850 to 1970. The sediment accumulation rates (SAR) showed a substantial increase in the 20th century. The increase in SAR corresponded with increases in erosional input from the watershed as represented by the increases in sediment concentrations of Ti and Al. The upper 20 cm of sediment, representing the last 150 years, also showed increases in C, N, P, and 15N. The increases in nutrient concentrations may be affected to various degrees by diagenetic reactions within the sediments, although the changes in concentrations also were marked by changes in the N:P ratio and in a qualitative change in the source of N as reflected in increasing δ15N. The diatoms showed modest changes in the 20th century, with increases in Asterionella formosa, Stephanodiscus hantzschii, and S. parvus. Pediastrum, a green alga, was well-preserved in the sediments and exhibited a sharp decline in relative abundance in the upper sediments. Total cyanobacteria, as represented by preserved akinetes, exhibited only minor changes in the last 1000 years. However, Aphanizomenon flos-aquae, a taxon which was formerly not present in the lake 150 years ago, but that now dominates the summer phytoplankton, has shown major increases over the past 100 years. The changes in sediment composition are consistent with activities including timber harvest, drainage of wetlands, and agricultural activities associated with livestock grazing, irrigated cropland, and hydrologic modifications.