ALBERT

Notes: 1. The restoration of deep lakes has traditionally focused on reducing the external phosphorus loading.2. Following the diversion of sewage effluent, that led to marked reductions in nutrient concentrations in its main inflow, Rostherne Mere has shown no reduction in phosphorus or chlorophyll a concentrations. A shallow lake upstream (Little Mere), however, has shown a marked response to effluent diversion.3. Nutrient budgets for Rostherne Mere reveal that sewage effluent was by far the most significant external source of total phosphorus and that diffuse drainage from the catchment was the most significant external source of dissolved inorganic nitrogen. Phosphorus loads from groundwater and a bird roost were insignificant. Internal sources of phosphorus were, however, considerable and were largely responsible for the observed delay in recovery.4. Phosphorus limitation of phytoplankton biomass may never be attainable because of substantial internal and diffuse sources of phosphorus, combined with a long retention time. Nitrogen is likely to be more important in limiting phytoplankton biomass. Control of diffuse nitrogen sources may therefore be more effective in the restoration of the deeper lakes of this region.

Notes: 1. Lake restoration from eutrophication often rests on a simple paradigm that restriction of phosphorus sources will result in recovery of former relatively clear-water states. This view has apparently arisen from early successful restorations of deep lakes in catchments of poorly weathered rocks. Lakes in the lowlands, however, particularly shallow ones, have proved less tractable to restoration. This study of three lowland lakes provides insights that illuminate a more complex picture.2. The lakes lie in a sequence along a single stream in a mixed urban and rural landscape. Severely deoxygenating effluent from an overloaded sewage treatment works was diverted from the catchment in 1991. Effects on two lakes, Little Mere (zmax 〈2 m) and Rostherne Mere (zmax 31 m) were followed until 2002. Mere Mere (zmax = 8 m), upstream of the former works, acted as a comparison for changes in water chemistry. Mere Mere showed no change in total phosphorus (TP), total inorganic nitrogen, or planktonic chlorophyll a concentrations. Increased winter rainfall was associated with higher winter soluble reactive phosphorus (SRP) and ammonium concentrations in its water.3. Little Mere changed from a deoxygenated, highly enriched, fishless system, with large populations of Daphnia magna Straus, clear water and about 40% aquatic plant cover, to a slightly less clear system following diversion. Daphnia magna was replaced by D. hyalina Leydig as fish recolonised. Spring peaks of chlorophyll a declined but summer concentrations increased significantly. Annual mean chlorophyll a concentrations thus showed no change. Submerged plants became more abundant (up to 100% cover), with fluctuating community composition from year to year. Summer release of SRP from the sediment was substantial and has not decreased since 1993. The summer phytoplankton was apparently controlled by nitrogen availability perhaps with some influence of zooplankton grazing. SRP was always very abundant. The lake appeared to have reached a quasi-stable state by 2002.5. Rostherne Mere showed a steady decline in TP and SRP concentrations following effluent diversion apparently as a result of steady dilution by water with lower phosphorus concentration. Decline in phosphorus concentrations was much less rapid than expected because of internal remobilisation from the hypolimnion and sediments. There have been no changes in chlorophyll a concentration or of nitrogen availability and by 2002 the phytoplankton probably remained limited by a combination of mixing, grazing and nitrogen.6. A seeming paradox is, thus, that immense changes in phosphorus budgets have shown no consequences for phytoplankton chlorophyll concentrations in either of the lakes, although the seasonal distribution has altered in Little Mere. Although these case studies deviate from others, for both shallow and deep lakes, they represent distinctive situations rather than undermining conventional models.

Notes: 1. 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 μg L−1 before loading reduction), subtropical to temperate (latitude: 28–65°), and lowland to upland (altitude: 0–481 m). Shallow north-temperate 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 μg 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 changes such as global warming. However, effects of global change are likely to run counter to reductions in nutrient loading rather than reinforcing re-oligotrophication.

Notes: 1. Hydrologically open lakes in temperate regions are not considered as sensitive to climate change as closed-basin lakes in semiarid regions.2. Oak Mere is an acid lake in lowland UK. It has no surface inflow or outflow but is connected to the regional groundwater. It is a high landscape, hydrologically open lake.3. This paper describes how Oak Mere responded to drought with rapid and substantial changes in acidity and nutrients. Decreases in pH and catchment-derived nutrients appear to be a response to the relative amount of catchment-derived minerals compared with those received in direct precipitation.4. The extreme sensitivity of the site appears to be due to negligible groundwater inputs, low buffering capacity and a short retention time.5. The paper demonstrates that high landscape sites may represent the most sensitive hydrologically open lakes to climate change. This not only has consequences for acidification and eutrophication studies. These sites may also be considered valuable in that they potentially offer an important new archive for palaeoclimate studies, with a sensitivity previously considered unavailable in low altitude temperate regions.

Notes: Abstract Top-down control of phytoplankton by zooplankton is possible through reductions in density of zooplanktivorous fish. Little Mere is a shallow lake where the effects of sewage effluent caused such a reduction. This allowed the large-bodied cladoceran, Daphnia magna Straus, to develop huge populations, preventing potentially large algal crops from developing. Subsequent diversion of the effluent is anticipated to lead to recovery of the fish community, reduced numbers of large-bodied grazers, and increased phytoplankton biomass. Whether the aquatic plant community, present in Little Mere, is resilient to such changes may depend upon whether cyanophytes are favoured, or not.

Notes: Abstract Previous work on a set of small lakes, of varying depth, themeresof North West England, has shown that nitrogen availabilitycontrols the summer phytoplankton populations in the deeperones(max depth〉3 m) and zooplankton grazing in shallow ones. Themeres have generally high total phosphorus concentrations andthismay be a natural phenomenon dependent on the localgeochemistry.Some anthropogenic eutrophication has occurred, however, andfroma chain of three meres, sewage effluent was diverted in 1991.Theupper lake, Mere Mere, lying above the point of discharge, hasnotchanged in any systematic way since effluent diversion. Themiddlelake, the very shallow Little Mere, has changed markedly inwaterchemistry but not fundamentally in ecosystem structure. It wasandremains a clear-water, macrophyte dominated lake. The thirdlake,the deep Rostherne Mere, has shown no response inchlorophyll a concentrations in four years since effluent diversionthough inthe past two years there appears to be a downward trend intotalphosphorus. The reasons for this are explored in terms of ourunderstanding of lake eutrophication. Comparisons are madewithWhite Mere, a deep groundwater fed lake with a long retentiontimeand a very high total phosphorus concentration. The deep meresmayadd a new dimension to our understanding of natural andanthropogenic eutrophication.