Many physiological processes in phytoplankton, including nutrient uptake, vary on a number of temporal scales. Experiments show that the daily cycle in irradiance affects nutrient uptake rates. We used a Droop-based model of resource competition to investigate how diel variability in nutrient uptake influences phytoplankton competition and community structure. The analytical approximation we derive shows that if nutrient uptake is light dependent, the minimum nutrient requirements and, hence, nutrient competitive abilities depend on light regime in a species-specific way. Consequently, daily variations in irradiance may slow rates of competitive exclusion or reverse the identity of the superior competitor but not allow stable coexistence. Irradiance-induced fluctuations in the maximum nutrient uptake rate of the superior competitor can lead to fluctuations in ambient nutrient concentration and an increase in the average nutrient concentration compared to constant light conditions. This can enhance nutrient use by inferior competitors. These results may be applicable to bacteria-phytoplankton nutrient competition as well. Depending on the costs and benefits of maintaining nutrient uptake in the dark, different strategies of nutrient use are optimal under different light regimes. Our results suggest that by mediating limiting nutrient use, fluctuations in irradiance may alter the structure of phytoplankton communities.In aquatic systems, physical forcing and biotic responses are tightly coupled (Steele and Henderson 1994). Major ecological interactions such as competition and predation are affected by environmental fluctuations. Light is an important environmental factor and an essential resource for phytoplankton that fluctuates on multiple temporal scales. Previously we have demonstrated that daily and seasonal light fluctuations significantly affect phytoplankton competition for light (Litchman and Klausmeier 2001). However, in many aquatic systems phytoplankton compete not for light but for nutrients. A substantial body of literature indicates that nutrient uptake rates depend on many environmental factors including light. It is, therefore, likely that light fluctuations can mediate nutrient competition among phytoplankton. Here we explore how variation in the uptake rates of the limiting nutrient modifies resource competition in phytoplankton.Numerous experimental studies have shown that uptake of many essential nutrients by phytoplankton depends on irradiance. Frequently, the uptake rates are lower in the dark. Riegman et al. (2000) showed that nitrogen (N) uptake of Emiliania huxleyi in the dark was 30% of the uptake rate in the light. Phosphorus (P) uptake in the dark was also lower than in the light, and the greatest difference between the uptake rates was for the lowest growth rate.The maximum nutrient uptake rate (V max ) usually exhibits a much stronger light dependence than the half-saturation

This introductory article of the special GAP issue gives an overview on general limnological characteristics of the prealpine Lakes Zürich and Lucerne and the alpine Lake Cadagno and reports on the specific situation of primary production parameters during the international GAP Workshop in mid September 1999. Furthermore, it describes methods used for water analysis and fieldwork in these lakes.A comparison of data related to primary production in the three lakes in September 1999 during stratification shows that (i) phytoplankton community structure varied considerably between the lakes. The dominating algae were Planktothrix rubescens in Lake Zürich, various chrysophytes and diatoms in Lake Lucerne, and Echinocoleum elegans in Lake Cadagno, (ii) the euphotic zone in Lake Lucerne was considerably deeper (app. 15 m) than in the other two lakes (app. 10 m), (iii) chlorophyll a standing crop was highest in mesotrophic Lake Zürich (August: 121 mg m -2 ), followed by oligotrophic Lake Lucerne (August: 75, September: 34 mg m -2 ) and mesotrophic Lake Cadagno (August: 33, September: 25 and 14 mg m -2 ), and (iv) areal primary production was highest in Lake Zürich (August: 105, September: 124 mg C m -2 h -1 ), followed by Lake Cadagno (August: 102, September: 52 mg C m -2 h -1 ) and Lake Lucerne (August: 90, September: 52 mg C m -2 h -1 ). Physiological parameters, determined in situ from P versus I relationships, showed a lower initial slope a in Lake Lucerne (August: 0.03, September: 0.02 mg C mg -1 chl a h -1 mmol -1 m 2 s) than in the other two lakes (Lake Zürich in August: 0.05, in September: 0.11; Lake Cadagno in August: 0.05, in September: 0.11 and 0.28 mg C mg -1 chl a h -1 mmol -1 m 2 s). Lake Zürich showed the lowest AN max (August: 2.6, September: 3.2 mg C mg -1 chl a h -1 , as compared to 5.9 -7.4 mg C mg -1 chl a h -1 in the Lakes Lucerne and Cadagno), while in Lake Cadagno the highest inhibitory effects of Cassimilation were found (highest slopes of inhibition b, 0.007 -0.011, as compared to 0.0003 -0.0026 in the other two lakes), due to a higher UV-exposure in this alpine lake.

Summary 1. Three common species of freshwater phytoplankton, the diatom Nitzschia sp., green alga Sphaerocystis schroeteri and cyanobacterium Phormidium luridum, were grown under contrasting daylengths [18 : 6 h light : dark cycles (LD) versus 6 : 18 h LD] and phosphorus (P) regimes (P‐sufficient versus 1 μm P). The rates of growth and photosynthesis, as well as growth efficiencies and pigment concentrations, were compared among treatments. 2. The growth and photosynthetic parameters of the three species depended on both P status and daylength in a species‐specific way. The responses to P limitation depended on daylength and, conversely, the responses to daylength depended on P status. 3. Growth rates and the maximum rates of photosynthesis (Pmax) of all species decreased under P limitation under both light regimes. However, the decrease of Pmax because of P limitation was greater under long daylength. The Pmax of the green alga S. schroeteri decreased the most (ca. sixfold) under P limitation compared with the other two species. The photosynthesis saturation parameter Ik also decreased under P limitation; the decline was significant in Nitzschia and Sphaerocystis. P‐limitation significantly increased photoinhibition (β) in Nitzschia and Sphaerocystis, but not in Phormidium. The excess photochemical capacity (the ratio of the maximum photosynthesis rate to the photosynthesis rate at the growth irradiance), characterising the ability to utilise fluctuating light, was significantly lower under P limitation. 4. The growth efficiency (growth rate normalised to daylength) declined with increasing daylength in all species. Under short daylength the cyanobacterium Phormidium had the lowest growth efficiency of the three species. 5. The cellular chlorophyll a concentration in both Nitzschia and Sphaerocystis was significantly higher under short daylength, but only under P‐sufficient conditions. In Nitzschia, under short daylength, P‐limitation significantly decreased cellular chlorophyll concentration. In contrast, P‐limitation increased cellular chlorophyll concentration in Sphaerocystis, but under long daylength only. The ratio of chlorophyll a to b in the green alga also declined under short daylength and under P‐limited conditions.

A detailed budget of the fluxes of bioavailable phosphorus (bio-P) was established for the ultra-oligotrophic Lake Brienz (Switzerland) and its catchment. Lake Brienz is a cold, deep oligotrophic peri-alpine lake that receives an annual load of approximately 300 kt of suspended sediments, mainly from two glacier-influenced rivers. The challenge was to overcome the associated high background of mineral-bound inorganic phosphorus (IP) of~200 t yr -1 that is mostly inaccessible to algae growth. The application of six complementary, independent datasets allowed a consistent balance of bio-P to be obtained. We made use of data on (a) the load imported by the contributing rivers, (b) net sedimentation from cores, (c) export of bio-P from catchment land to the surface waters estimated by a GIS model, (d) the downward flux of bio-P through the water column from sediment traps, (e) primary production, and (f) the mineralization rate of organic material from the consumption of oxidants in the uppermost sediment of the lake. The average bio-P load estimated from import measurements and net sedimentation is 7.0 t yr -1 with an error of about 10 %: An estimated 5.4 t yr -1 enters by way of the two main rivers (including 0.9 t yr -1 from sewage treatment plants), 1.2 t yr -1 from the remaining catchment (including 0.4 t yr -1 from sewage treatment plants that are diverted directly into the lake), and~1 t yr -1 from atmospheric deposition. Approximately 2 t of bio-P are retained annually in the sediments of the upstream dams and thereby withheld from downstream Lake Brienz. The maximum eutrophication of the lake in the late 1970s and the subsequent re-oligotrophication can be attributed to the loads of urban wastewater. The drop in biological productivity since the late 1970s is consistent with the decrease of bio-P fluxes archived in the sediment, the record of the sewage treatment plant outflows and the few occasional in-situ observations.

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