Water level as the mediator between climate change and phytoplankton composition in a large shallow temperate lake

Hydrobiologia - We studied the effect of water level changes in Lake Võrtsjärv (270 km2, mean depth 2.8 m) on the abundance and composition of phytoplankton based on a 35-year database....     

The temporal and spatial patterns of protozooplankton abundance in a eutrophic temperate lake

The temporal and spatial distribution of planktonic protozoa of Esthwaite, a eutrophic lake, was investigated at 7–10 day intervals between February to October 1988. Sarcodine protozoa were of little significance, the plankton was dominated by ciliates and flagellates. Ciliate density peaked in late May to early June with densities reaching 9.2 × 10³ 1^-1. There was considerable variation in spatial distribution and greatest species diversity occurred in March/April. After the establishment of summer stratification the planktonic ciliates were confined to water of >25% oxygen saturation in the water column. Oligotrichs, particularly the genus Strombidium and tintinnids, and peritrichs dominated the ciliate assemblages. There was no correlation between chlorophyll a concentrations and ciliate numbers, but a correlation was apparent between ciliate numbers and flagellate density. There were significant differences between the protozooplankton communities at the different sampling sites in the lake.     

The relationship between phytoplankton biovolume and chlorophyll in a deep oligotrophic lake: decoupling in their spatial and temporal maxima

The seasonal distributions of phytoplankton biovolume and chlorophylla content were monitored for 14 months in a deep oligotrophic,high mountain lake (Redó, Pyrenees). An allometric relationshipof chlorophyll with biovolume was found throughout the periodstudied, with a correlation coefficient of 0.66. However, therelationship changed with season and the taxonomic compositionof the phytoplankton. Both parameters showed a similar seasonalpattern, but differences in space and time were observed. Thechlorophyll maximum was recorded deeper and later than thatof phytoplankton biovolume. While the biovolume maximum wasrelated to an improvement in conditions for growth (nutrientinput during column mixing periods), and reflected an increasein biomass, the chlorophyll maximum was related to changes incell pigment content, and to spatial or successional trendsin species dominance. Flagellated chrysophytes predominatedat the chlorophyll maxima. Chlorophyll content per unit of phytoplanktonbiovolume fluctuated greatly throughout the year, dependingon light intensity, temperature and phytoplankton composition.Of the main groups of phytoplankton in the lake, the dinoflagellates,which dominated the summer epilimnion phytoplankton community,recorded the lowest pigment content per biovolume (which isconsistent with their size). Higher chlorophyll contents perbiovolume were found in the deep hypolimnion and during thewinter cover period associated with small cells such as somespecies of chlorococcales chlorophytes. When flagellated chrysophyteswere predominant, a broad range of chlorophyll values per biovolumewas found and there was no significant correlation between thetwo biomass indices. These findings reaffirm the need to treatphytoplankton biomass estimates with caution, in particularwhen conducting primary production studies. While our resultsshow that changes in chlorophyll content per cell occur as aphotoacclimation response along a vertical profile, they alsopoint out a component of the successional trends which appearin a phytoplankton growth phase in a lake.     

Association between temporal and spatial beta diversity in phytoplankton

The rates of temporal and spatial species turnover have been compared in different organisms and scales, revealing that both are not independent but, rather, associated. However, the knowledge is limited for the association between spatial turnover and temporal turnover. Here, we performed two investigations of the phytoplankton composition in the lakes of the Yangtze River catchment in China in the spring and summer of 2012, which covered regional spatial scale and two‐season temporal scale. We analysed the association between temporal and spatial species turnover in phytoplankton. The results showed that 1) the two‐season temporal turnover of phytoplankton varied based on the mean values and the coefficient of variance of environmental variables, and pH was the most important variable negatively affecting the temporal turnover; 2) the spatial beta diversity of phytoplankton in summer was higher than that in spring, and the distance decay pattern was significant in summer, but not in spring; 3) the variation in spatial turnover in spring and summer was attributed to the primary environmental variables (nitrogen, phosphorus and underwater available light) and broader‐scale spatial variables; 4) the proportion of jointly explained variation of spatial Bray–Curtis dissimilarity by the environment and space increased from ~38% (spring) to ~55% (summer), which was mainly due to the variation in spatially structured environmental variables during the two‐season temporal turnover, such as pH and ion concentrations; 5) the community compositions in summer were more similar between the lakes with similar two‐season temporal turnover. These results indicate that the spatial turnover of phytoplankton composition in summer was partially predetermined by the variation in environmental variables and phytoplankton composition during the process of two‐season temporal turnover, and highlight the understanding of temporal variations in spatial beta diversity as well as the underlying assembly mechanisms in phytoplankton.     

Effects of recent climate change on phytoplankton phenology in a temperate lake

1. A number of long-term studies have shown that spring biological events have advanced in recent decades and that this is a response to climate change. In lentic systems, changes in phytoplankton phenology have been attributed to various directly climate-related processes including changes in the onset and duration of thermal stratification, earlier ice-break up and increased water temperature. Both indirect climatic drivers and non-climate drivers such as elevated grazing pressure and nutrient enrichment can also affect phenology.
2. This study investigated whether phenological trends in phytoplankton could be detected in a relatively short time series in a shallow, ice-free, polymictic lake with a high annual discharge and whether any such trends could be causally explained.
3. It was found that the centre of gravity of the spring chlorophyll a bloom advanced significantly by 1.6 days per year over a 15-year period. This was accompanied by a significant increase in water temperature of 0.12 °C per year which is high compared to published rates of change over longer time series. No direct effects of ice cover, stratification or water discharge rates could be linked to the advancement of the spring bloom. Instead, the shift in timing was attributed to an advance in the timing of the dominant spring diatom, Aulacoseira spp., instigated by a temperature-driven increase in replication rate leading to an earlier onset of silica (SiO₂) limitation.     

Colorful niches of phytoplankton shaped by the spatial connectivity in a large river ecosystem: a riverscape perspective

Large rivers represent a significant component of inland waters and are considered sentinels and integrators of terrestrial and atmospheric processes. They represent hotspots for the transport and processing of organic and inorganic material from the surrounding landscape, which ultimately impacts the bio-optical properties and food webs of the rivers. In large rivers, hydraulic connectivity operates as a major forcing variable to structure the functioning of the riverscape, and--despite increasing interest in large-river studies--riverscape structural properties, such as the underwater spectral regime, and their impact on autotrophic ecological processes remain poorly studied. Here we used the St. Lawrence River to identify the mechanisms structuring the underwater spectral environment and their consequences on pico- and nanophytoplankton communities, which are good biological tracers of environmental changes. Our results, obtained from a 450 km sampling transect, demonstrate that tributaries exert a profound impact on the receiving river's photosynthetic potential. This occurs mainly through injection of chromophoric dissolved organic matter (CDOM) and non-algal material (tripton). CDOM and tripton in the water column selectively absorbed wavelengths in a gradient from blue to red, and the resulting underwater light climate was in turn a strong driver of the phytoplankton community structure (prokaryote/eukaryote relative and absolute abundances) at scales of many kilometers from the tributary confluence. Our results conclusively demonstrate the proximal impact of watershed properties on underwater spectral composition in a highly dynamic river environment characterized by unique structuring properties such as high directional connectivity, numerous sources and forms of carbon, and a rapidly varying hydrodynamic regime. We surmise that the underwater spectral composition represents a key integrating and structural property of large, heterogeneous river ecosystems and a promising tool to study autotrophic functional properties. It confirms the usefulness of using the riverscape approach to study large-river ecosystems and initiate comparison along latitudinal gradients.     

Spatial and temporal dynamics of the steady-state phytoplankton assemblages in a temperate shallow hypertrophic lake (Lake Manyas,Turkey)

Spatial and temporal dynamics of phytoplankton biomass and species composition in the shallow hypertrophic Lake Manyas, Turkey, were studied biweekly from January 2003 to December 2004 to determine steady-state phases in phytoplankton assemblages. Steady-state phases were defined when one, two or three coexisting species contributed to at least 80% of the standing biomass for at least 2 weeks and during that time the total biomass did not change significantly. Ten steady-state phases were identified throughout the study peiod. During those periods, Achnanthes microcephala (Kützing) Cleve twice dominated the phytoplankton biomass alone and contributed to more than 50% of the total biomass in seven phases. Microcystis aeruginosa (Kützing) Kützing, Anabaena spiroides Klebahn, Cyclotella stylorum Brightwell, Pediastrum boryanum (Turpin) Meneghini and Phacus pusillus Lemmermann were also represented once in steady-state phytoplankton assemblages. A. microcephala was dominant usually during cold periods of the year, while M. aeruginosa and A. spiroides were usually dominant in warm seasons. The total number of species showed a clear decrease during steady-state phases at all stations. All stations were significantly different in terms of the measured physical and chemical parameters (P < 0.05) and phytoplankton biomass (F = 117, P < 0.05).     

Zooplankton and ichthyoplankton communities in a temperate estuary: spatial and temporal patterns

Zooplankton and ichthyoplankton assemblages were studied fromJanuary 2003 to June 2004 in a temperate shallow estuary (Mondegoestuary, Portugal). Monthly sampling was performed at five stationsat high and low tides, with subsurface tows with 335 and 500µm mesh Bongo nets. Analysis of variance (ANOVA) showeda significant effect (P < 0.05) of the mesh size of the neton the abundance of main zooplankton groups. On average, theabundance of the 500 µm taxocenosis was 67 and 102 timeslower than the 335 µm taxocenosis at high and low tidesrespectively, especially in the south arm. More than 80 specieswere identified in the zooplankton samples. The upper reachesof northern arm are dominated by freshwater crustacean mesozooplanktonlike Acanthocyclops robustus and Diaptomus spp. and the cladoceransDaphnia, Ceriodaphnia and Bosmina, often being codominant. Inthe southern arm, the resident estuarine copepod Acartia tonsawas dominant, eventually being the most abundant taxon. Marinereaches of estuary are usually dominated by the marine calanoidcopepods Acartia clausi and Temora longicornis and the siphonophoresMuggiaea atlantica. Concerning the ichthyoplankton, this wasdominated by the larvae of estuarine resident species, mainlyPomatoschistus sp., and eggs of Engraulis encrasicolus. Theabundance of Pomatoschistus sp. larvae was positively correlatedwith water temperature. Statistical analysis (canonical correspondenceanalysis) used to determine the spatiotemporal structure ofthe zooplankton assemblages and its correlation with environmentalvariables showed that salinity and temperature were the mainfactors influencing the distribution of zooplankton. The resultsobtained also showed that abundance was strongly influencedby the hydrological circulation pattern and direct or indirecthuman impacts that occur in each arm of the estuary. This article was presented at Plankton Symposium III, held atFiguera da Foz, portugal, between 17 and 20 March 2005, underthe auspices of the University of Coimbra and the Universityof Aveiro, and coordinated by Mário Jorge Pereira andUlisses M. Azeiteiro.     

Interactions between phytoplankton and zooplankton in a fertilized lake

The limnology of an oligotrophic lake, Langvatn, situated in Trøndelag county in Central Norway, has been studied for five years (1974–1978). In two years, 1975 and 1976, the lake was fertilized with a general fertilizer to change feeding conditions for the zooplankton. Mean phytoplankton biomass in the epilimnion and primary production for the years (1974–1978) were 417, 618, 1370, 607 and 779 mg m⁻³ and 10.6, 22.2, 49.0, 26.8 and 17.7 g C m⁻² yr⁻¹, respectively. Cladocerans were the dominant herbivore group in 1974 and 1975 and Rotifera in the next three years. The main difference in the interaction between phytoplankton and zooplankton occurred when cladoceran dominance gave way to rotifer dominance. Heavy phytoplankton grazing by cladocerans in 1974 and 1975 stabilized the biomass and maintained it at a low level, which also resulted in a relatively low primary production. The rotifer-dominated community during the years 1976–1978 did not possess the ability to maintain a stable level of algal biomass. Primary production was also relatively high during these years.     

Effects of lake mixing with an axial flow pump on water chemistry and phytoplankton

This paper describes the effect of total lake mixing with an axial flow (Garton) pump on the limnology and phytoplankton of two Oklahoma lakes.The Garton pump destratified Ham's Lake (40 ha) in 3 days. Except for one small isolated basin, Ham's Lake remained completely destratified for the rest of the summer. Algal biomass declined, numbers of species of green algae increased, but numbers of species of blue-green algae did not decrease as expected. After destratification, pH remained high (> 8), carbonate alkalinity was observed and reactive phosphate was undetectable.An axial flow pump increased the heat content of Arbuckle Lake (951 ha) and caused the lake to destratify about one month earlier than usual. Increasing the heat content of the lake did not affect the concentration of most water quality parameters or the biomass of algae.     

Within‐lake distribution patterns of fish assemblages: the relative roles of spatial,temporal and random environmental factors in assessing fish assemblages using gillnets in a large and shallow temperate lake

In this study, the relative role of spatio‐temporal factors and associated environmental variables (water transparency and temperature) were quantified in relation to gillnet samples of fishes in a large and shallow lake (Lake Balaton, Hungary). Most of the variance (56·1%) in the relative abundance data (%) was related to the vertical segregation of fishes. This gradient substantially affected the catch per unit effort (CPUE) by number of the dominant species, the surface‐oriented bleak Alburnus alburnus and the benthic common bream Abramis brama. It also influenced total CPUE, mean fish mass and species richness and diversity. At the lake level, horizontal habitat heterogeneity (i.e. littoral v. offshore) accounted for only 8·3% of the total variance in relative abundance data, but was important in structuring the CPUE of the ruffe Gymnocephalus cernua and the pikeperch Sander lucioperca. The longitudinal environmental gradient (i.e. lake basin), year and season of sampling, water transparency and temperature had significant effects on relative abundance only at the habitat level, but were also important components of variability of CPUE in some species at the lake level. As sampling schemes need to consider the main gradients in fish assemblage distributions, the use of surface and pelagic gillnets should be more intensively incorporated in the study and monitoring of fish assemblages in shallow lakes and lake habitats.     

Contrasting roles of water chemistry, lake morphology, land-use, climate and spatial processes in driving phytoplankton richness in the Danish landscape

Understanding of the forces driving the structure of biotic communities has long been an important focus for ecology, with implications for applied and conservation science. To elucidate the factors driving phytoplankton genus richness in the Danish landscape, we analyzed data derived from late-summer samplings in 195 Danish lakes and ponds in a spatially-explicit framework. To account for the uneven sampling of lakes in the monitoring data, we performed 1,000 permutations. A random set of 131 lakes was assembled and a single sample was selected randomly for each lake at each draw and all the analyses were performed on permuted data 1,000 times. The local environment was described by lake water chemistry, lake morphology, land-use in lake catchments, and climate. Analysis of the effects of four groups of environmental factors on the richness of the main groups of phytoplankton revealed contrasting patterns. Lake water chemistry was the strongest predictor of phytoplankton richness for all groups, while lake morphology also had a strong influence on Bacillariophyceae, Cyanobacteria, Dinophyceae, and Euglenophyceae richness. Climate and land-use in catchments contributed only little to the explained variation in phytoplankton richness, although both factors had a significant effect on Bacillariophyceae richness. Notably, total nitrogen played a more important role for phytoplankton richness than total phosphorus. Overall, models accounted for ca. 30% of the variation in genus richness for all phytoplankton combined as well as the main groups separately. Local spatial structure (<30 km) in phytoplankton richness suggested that connectivity among lakes and catchment-scale processes might also influence phytoplankton richness in Danish lakes.     

Nutrient limitation of autotrophic and mixotrophic phytoplankton in a temperate and tropical humic lake gradient

Nutrient enrichment experiments were carried out in three tropical(once) and three temperate (twice) lakes differing in humiccontent in order to examine whether there was a relationshipbetween the limiting nutrient for algal growth [nitrogen (N)or phosphorus (P)] and humic content, and whether the prevailinglimitation was connected to the relative abundance of autotrophicand phagotrophic phytoplankton (mixotrophs). In both climaticregions, there was a stronger tendency for total phytoplanktonbiomass accumulation to be N limited in lakes with a high humiccontent. However, in contrast to what we expected, there wasno tendency for the mixotrophs to be more favored by the additionof N than of P. In the temperate lakes, the relative abundanceof mixotrophs increased in the treatments receiving N or P separatelyor no nutrients (control) when exposed to a high light availability.In the following year, when the light availability was low,the mixotrophs increased relative to the obligate autotrophsin all treatments, irrespective of nutrient addition. Possibly,this was a result of their ability to supplement photosynthesiswith the ingestion of prey. The results indicate that mixotrophyis an advantageous strategy when the availability of light and/ornutrients is low.     

Community structure in north temperate ants: temporal and spatial variation

Summary Ant communities in Vermont and New York woods were sampled in four time periods to determine species composition, relative abundances, and nest locations in space. The Vermont community was richer, containing more species and higher nest densities than New York. Both communities followed the geometric distribution of species abundances, suggesting that a single resource was mediating competition. The resource most clearly implicated was suitable nest sites, principally pre-formed plant cavities. Nonrandom species associations, underdispersion in every season, and the occurrence of incipient nests overwintering aboveground all implicated shortage of such cavities. Furthermore, microhabitat differences which produce suitable nest sites occur over a very small scale in these communities.     

Temporal and spatial variations in phytoplankton carbon isotopes in a polymictic subtropical lake

Stable carbon isotopes (¹³C) were determined for phytoplanktonand dissolved inorganic carbon (DIC) from Lake Apopka, a shallow,polymictic and hypereutrophic lake in Florida, USA. Bulk planktondominated by pico- and nanqanobacteria were enriched in ¹³(–13.1± 1.1%) as a result of assimilation of extremely ¹³C-richDIC (¹³C = 9.6 ± 3.0%). Diatoms (Aulacoseira spp.) hada ¹³C of –14.3 ± 0.6% that was slightly more negativethan that of small cyanobacteria. Meroplanktonic diatoms hada ¹³C (–13.6 ± 1.8%), similar to their planktoniccounterparts. The ¹³C of a colonial cyanobacterium (Microcystisincerta) was exceptionally heavy (–3.0 ± 1.0%)and attributed to localized carbon limitation. Seasonal variationin ¹³C of bulk plankton was small (4%) relative to reports forother lacustrine systems No difference in the ¹³C of bulk planktonhorn surface water between stratified and non-stratified periodswas found. No measurable changes in ¹³C of bulk plankton wereindicated in light and dark incubation experiments Frequentwind mixing of the water column, high DIC concentration, andconsistently high lake productivity were used to explain thetemporal and spatial isotope consistency of phytoplankton inthis lake.     

Manipulation of chemistry and phytoplankton by hydrological intervention: a whole lake experiment in the northern Netherlands

SUMMARY. 1. The small Lake Negenmad, in the nature reserve 'De Oude Venen', was isolated from the canals and lakes of the Frisian lake system. Its chemistry and phytoplankton were monitored 1 year before (1984) and 2 years after (1985–86) isolation. These characteristics were compared with those of the adjacent Lake Veertigmad which was not manipulated.
2. The hydrological intervention prevented the inflow of chloride-rich IJsselmeer water into Lake Negenmad and made its water table more dependent on evapotranspiration, precipitation and upwelling water. In comparison with the unmanipulated lake. Lake Negenmad became less saline (c. 50%), more humic (c. 50%) and total dissolved (<0.2,μm) iron concentration increased dramatically.
3. During the 2 years of damming no marked differences in the total nutrient concentrations of either lake were observed. However, the maximum phytoplankton density in the dammed lake was half that in the unmanipulated lake. The lower density may have been caused by low P availability after formation of humus-iron phosphate species.
4. Preventing the inflow of water from the Frisian lake system favoured the occurrence in Lake Negenmad of flagellated species (Chrysophyceae, Cryptophyceae) at the expense of filamentous and nuisance-causing cyanobacteria (Oscillutoria, Anabaena, Aphanizomenon spp.).
5. The implications of these results for water quality management of reservoirs in peaty areas are briefly discussed.     

Spatial and temporal variations in aluminum chemistry of a dilute,acidic lake

Elevated concentrations of Al have been observed in acidic surface waters. An assessment of the chemistry of aqueous Al is of interest because of its role as a toxicant to aquatic organisms, a pH buffer, and an adsorbent of orthophosphate and organic carbon. In this investigation we evaluated the spatial and temporal fluctuations of Al forms in an acidic drainage lake.High concentrations of NO ₃ ^– (51.0 ± 11 mol l^–1), H⁺ (14.9 ± 3.5 mol l^–1), and Al (19.6 ± 3.5 mol l^–1) were introduced to Dart's Lake through drainage water during the snowmelt period. During low flow periods microbially mediated depletions of nitrate served to neutralize H⁺ and aluminum base neutralizing capacity. Thus in Dart's Lake, NO ₃ ^– transformations were extremely important in regulating short-term changes in pH and subsequent changes in the inorganic forms of Al. During stratification periods Al appeared to be non-conservative within the lake system. Although we know very little about the character and transformations of alumino-organic solutes, these substances were correlated with dissolved organic carbon (DOC) concentrations. Alumino-organic substances appear to be introduced to the lake from both drainage water and sediments.     

High-frequency metabolism study in a large and shallow temperate lake reveals seasonal switching between net autotrophy and net heterotrophy

Respiratory CO₂ release from inland waters is a major process in the global carbon cycle, retaining more than half of the carbon flux from terrestrial sources that otherwise would reach the sea. The strongly lake type-specific balance between primary production and respiration determines whether a lake acts regionally as a net sink or source of CO₂. This study presents two-year (2009, 2010) results of high-frequency metabolism measurements in the large and shallow polymictic eutrophic Lake V?rtsj?rv (area 270?km²; mean depth 2.8?m). We estimated the net ecosystem production (NEP), community respiration (R) and gross primary production (GPP) from continuous measurements of oxygen, irradiance, wind and water temperature. A sinusoidal model fitted to the calculated metabolic rates showed the prevalence of net autotrophy (mean GPP:R?>?1) from early spring until August/September, whereas during the rest of the year heterotrophy (mean GPP:R?2 neutral on an annual basis. Community respiration lagged behind GPP by approximately 2?weeks, which could be explained by the bulk of the phytoplankton biomass accounted for by filamentous cyanobacteria that are considered mostly inedible to zooplankton, and the seasonally increasing role of sediment resuspension. In the warmer year 2010, the seasonal peaks of GPP, R and NEP were synchronously shifted nearly 1?month earlier compared with 2009. The strong stimulating effect of temperature on both GPP and R and its negative effect on NEP revealed by the multiple regression analysis suggests increasing metabolic rates and increasing heterotrophy in this lake type in a warmer climate.     

Net pelagic heterotrophy in mesotrophic and oligotrophic basins of a large, temperate lake

Understanding the effects of trophic status and dissolved organic carbon concentration (DOC) on lake carbon cycling is essential for accurate ecosystem carbon models. Using isotopically labelled substrates we assessed spatial and temporal variability in bacterial respiration (BR) and algal primary production (PP) in two trophically, morphometrically and hydrologically different basins in Loch Lomond, a large temperate lake in Scotland. GIS modelling was used to construct a whole lake balance for bacterial production/respiration and PP, and from this the proportion of heterotrophy fuelled by allochthonous carbon was estimated. We tested the hypotheses that trophic status and DOC concentration affect the balance between PP and BR and examined which is the more significant driving factor. Additionally we estimated the percentage of BR that is fuelled by terrestrial carbon. PP varied seasonally and showed inter-basin homogeneity. BR was greatest in the mesotrophic south basin in autumn, which corresponded to measured peak DOC input, though over an annual cycle no relationship was observed between BR and DOC concentration. The PP:BR ratio was 0.37 ± 0.30 and 0.3 ± 0.45 in the north and south basins, respectively, assuming a bacterial growth efficiency of 0.1. We have found that allochthonous carbon potentially supports a substantial quantity of pelagic production, even during periods of high photosynthesis. Less productive systems are thought to be dominated by heterotrophic processes. However, we have found that the mesotrophic basin of a large lake to be as heterotrophic as its neighbouring oligotrophic basin, an observation that has implications for our understanding of modelling of the role of lakes in linking the terrestrial-atmospheric carbon cycle.     

Reconstructed long-term time series of phytoplankton primary production of a large shallow temperate lake: the basis to assess the carbon balance and its climate sensitivity

Aiming at building the carbon budget for further climate change impact research in the large and shallow northern temperate Lake Võrtsjärv, the present paper focuses on reconstructing the full phytoplankton primary production (PP) data series for the lake for the period of 1982–2009 covered by disconnected measurements, and testing the uncertainties involved both in the PP measurements and bio-optical modelling. During this 28-year period, in situ PP was measured in Võrtsjärv in 18 years with ¹⁴C-assimilation technique. We reconstructed the full time series using a semi-empirical PP simulation model based on continuously measured PAR irradiance and interpolated values of monthly measured chlorophyll a (C _chl). The modelling results, which proved highly reliable during the calibration phase, correlated rather weakly with the annual PP estimates for the 18 years, which were based on 2-h incubations at midday, 1–2 times per month. Being based on continuous irradiance data, the modelled PP can be considered more reliable than the sparse measurements, especially for short to medium term studies. We demonstrate that in the long-term, the bio-optical method can be biased if changes in water colour or water level alter the light climate causing adaptive responses in the cellular chlorophyll content of light-limited phytoplankton.