SUMMARY1. This synthesis examines 35 long-term (5-35 years, mean: 16 years) lake re-oligotrophication studies. It covers lakes ranging from shallow (mean depth <5 m and/or polymictic) to deep (mean depth up to 177 m), oligotrophic to hypertrophic (summer mean total phosphorus concentration from 7.5 to 3500 lg L )1 before loading reduction), subtropical to temperate (latitude: 28-65°), and lowland to upland (altitude: 0-481 m). Shallow northtemperate lakes were most abundant. 2. Reduction of external total phosphorus (TP) loading resulted in lower in-lake TP concentration, lower chlorophyll a (chl a) concentration and higher Secchi depth in most lakes. Internal loading delayed the recovery, but in most lakes a new equilibrium for TP was reached after 10-15 years, which was only marginally influenced by the hydraulic retention time of the lakes. With decreasing TP concentration, the concentration of soluble reactive phosphorus (SRP) also declined substantially. 3. Decreases (if any) in total nitrogen (TN) loading were lower than for TP in most lakes. As a result, the TN : TP ratio in lake water increased in 80% of the lakes. In lakes where the TN loading was reduced, the annual mean in-lake TN concentration responded rapidly. Concentrations largely followed predictions derived from an empirical model developed earlier for Danish lakes, which includes external TN loading, hydraulic retention time and mean depth as explanatory variables. 4. Phytoplankton clearly responded to reduced nutrient loading, mainly reflecting declining TP concentrations. Declines in phytoplankton biomass were accompanied by shifts in community structure. In deep lakes, chrysophytes and dinophytes assumed greater importance at the expense of cyanobacteria. Diatoms, cryptophytes and chrysophytes became more dominant in shallow lakes, while no significant change was seen for cyanobacteria. 5. The observed declines in phytoplankton biomass and chl a may have been further augmented by enhanced zooplankton grazing, as indicated by increases in the zooplankton : phytoplankton biomass ratio and declines in the chl a : TP ratio at a summer mean TP concentration of <100-150 lg L )1 . This effect was strongest in shallow lakes. This implies potentially higher rates of zooplankton grazing and may be ascribed to the observed large changes in fish community structure and biomass with decreasing TP contribution. In 82% of the lakes for which data on fish are available, fish biomass declined with TP. The percentage of piscivores increased in 80% of those lakes and often a shift occurred towards dominance by fish species characteristic of less eutrophic waters. 6. Data on macrophytes were available only for a small subsample of lakes. In several of those lakes, abundance, coverage, plant volume inhabited or depth distribution of submerged macrophytes increased during oligotrophication, but in others no changes were observed despite greater water clarity. 7. Recovery of lakes after nutrient loading reduction may be confounded by concomitant environmental cha...

Organic matter and its carbon and nitrogen isotopic composition were measured in sequential sediment trap and core samples from the Rochester Basin of Lake Ontario to evaluate their usefulness in reconstructing historic changes in lake productivity. The greatest flux of organic matter from the epilimnion occurred during late summer and coincided with whiting events, indicating that calcite precipitation is an effective mechanism for sedimenting organic matter. Carbon isotopes of organic matter were low prior to the onset of stratification, increased to maximum values in late summer, and then decreased following fall overturn. This pattern is controlled mainly by the timing of stratification and primary productivity, which preferentially removes "COZ from the epilimnion. The physiological effect of decreased carbon isotopic fractionation with decreasing supplies of [CO,],,, may have also contributed to increased PC orgc' Nitrogen isotopes showed a seasonal pattern opposite to that of carbon, whereby S"N values were low during the summer stratified period and high for the remainder of the year. Seasonal variability in S'INorgN probably reflects changes in the source of sedimented organic particles, which is dominated by isotopically depleted phytodetritus during the stratified period and isotopically enriched organic matter from heterotrophic or detrital sources during the mixed period. A comparison of organic carbon accumulation rates and 6'&, between sediment cores collected in 1987 and 1993-1994 confirms earlier predictions that diagenetic processes reduce the mass accumulation of organic carbon in the zone of oxic pore waters, but will not change the S"C,,,,,. values. All cores analyzed for PC,,,, display the reproducible pattern of a progressive increase in the 19OOs, peaking in the early to mid-1970s, and then decreasing to the present. This pattern matches the historical trends of phosphorus loading to the basin, suggesting that 613C of organic carbon is a reliable proxy for paleoproductivity and responds to spring phosphorus supplies in the water column. The 6"N of sedimentary organic matter increased linearly from 1840 to 1960 at a rate of 0.3%0 per decade, and remained relatively constant thereafter except for an increase in the upper few centimeters of sediment. The increase in @N,,,,, reflects a combination of factors, including early forest clearance by Europeans, increased sewering by municipalities after 1940, and increased nitrate utilization as productivity increased in the lower Great Lakes. Increased rates of denitrification in the central basin of upstream Lake Erie from the 1930s to the early 1970s may have also contributed to the rise in P5N,,,, values. Proxy indicators of past changes in primary productivity have long been sought in both marine and lacustrine sedimentary records. In the Laurentian Great Lakes, historical information on nutrient loading and productivity is limited before 1964 (Stevens and Neilson 1987), and thus paleolimnological studies have been used to infer nut...

Lacustrine productivity in Lake Erie was reconstructed using measurements of V3C of sedimented organic C (S13C,,,) and non-apatite inorganic P (NAIP) in three cores. Inferred changes in lacustrine productivity were related to historic changes in phosphorus loading. P loading increased slowly after forest clearance and early settlement in the late 18OOs, then increased exponentially from the late 1940s to early 197Os, and finally decreased after the mid-1970s in response to management practices implemented to improve water quality. Similarly, the sediment accumulation rate of NAIP increased rapidly during the 1940s and 195Os, peaked during the late 1960s and early 197Os, and then decreased after the mid-1970s. The signal of 613C,, normalized to account for the historic 1.4o/oo depletion in V3C of atmospheric CO, mirrors that for accumulation of NAIP in sediments. Our results show that paleoproductivity peaked in the lower Great Lakes during the mid-1970s but has since declined as a result of P abatement programs. Inferred trends for NAIP and 613C Orgc are those predicted in P-limited lacustrine systems and demonstrate that the carbon isotopic ratio of organic C can be used as a proxy for paleoproductivity in large, deep lakes.

We studied water, sediment trap, and core samples from eastern Lake Ontario to reconstruct the factors controlling the biologically induced production and sedimentation of calcite during so-called whiting events. Calcite accumulation and its isotopic composition are controlled by a complex set of interrelated factors, including temperature, primary productivity, and the abundance of pica-cyanobacteria during the stratified period. Calcite precipitation is highly correlated to lake temperature, because physical and biological factors interact to produce conditions favorable for whitings during warm years when the lake stratifies early in the seasonal cycle. Carbonate stratigraphies in multiple cores from eastern Lake Ontario revealed similar patterns of historical variation in percent carbonate. An exponential rise in carbonate accumulation occurred in nine cores after 1930, culminating in peak values in the early 1980s. This rise was related to historic increases in primary productivity resulting from increased phosphorus loading to Lake Ontario. Superimposed upon this rise were four peaks (centered on 1940-1942, 1957-1961, 1971-1977, and 1983) that correlate with maxima in summer air-temperature anomalies from the Great Lakes region and with strong El Niiio events. These peaks are also associated with maxima in S'C values and minima in S180 values of carbonate, lending support to our model that more calcite is precipitated with higher 6'C values during warm years when thermal stratitication occurs early in the seasonal cycle. Beginning in the mid-1980s calcite accumulation and its 6°C ratio began to decrease, suggesting a reduction in primary productivity in surface waters, probably related to lower phosphate concentrations in epilimnetic waters of Lake Ontario during the stratified period. Reduced summer P loading may be explained either by a lagged response to P abatement measures that began in the late 1970s or by decreased P loading from upstream Lake Erie beginning in the late 1980s as a result of the establishment of filter-feeding zebra mussels

Weekly water samples were taken to measure stable isotope composition (d 13 C and d 15 N) of particulate organic matter (POM) in Lake Wauberg, Florida, from June 1994 to May 1995. The average d 13 C of POM was 219.3%, consistent with an autochthonous origin from phytoplankton production, and exhibited a seasonal pattern that coincided with changes in water temperature, pH, CO 2 concentration, and phytoplankton biomass in the surface water. The 13 C enrichment in POM was attributed to reduced isotope fractionation due to carbon (C) limitation and the use of an isotopically heavy dissolved inorganic carbon pool supported mainly by atmospheric invasion and anaerobic respiration. Intermittent declines in d 13 C of POM were related to the frequent collapses of phytoplankton blooms and increases in CO 2 concentration resulting from both increased community respiration and terrestrial loading. Average d 15 N of POM was 1.3% and varied within a narrow range (20.1% to 2.5%). No significant correlation between phytoplankton biovolume and the d 15 N of POM was found. The low d 15 N is indicative of strong N 2 fixation, which is in line with the low concentration of dissolved inorganic nitrogen and the presence of high biovolume of N 2 -fixing cyanobacteria in the surface water. This study suggests that stable C isotopes are good proxies for surface water CO 2 concentration and primary production, while stable N isotopes can be used to indicate N 2 fixation.