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In lakes, trophic change and climate change shift the relationship between nutrients and physical factors, like temperature and photoperiod, and interactions between these factors should affect the growth of phytoplankton species differently. We therefore determined the relationship between P-limited specific growth rates and P-quota (biovolume basis) of Stephanodiscus minutulus and Nitzschia acicularis (diatoms) at or near light saturation in axenic, semi-continuous culture at 10, 15 and 20 °C and at 6, 9 and 12 h d(-1) photoperiod. Photoperiod treatments were performed at constant daily light exposure to allow comparison. Under these conditions, we also performed competition experiments and estimated relative P-uptake rates of the species. Temperature strongly affected P-limited growth rates and relative P uptake rates, whereas photoperiod only affected maximum growth rates. S. minutulus used internal P more efficiently than N. acicularis. N. acicularis was the superior competitor for P due to a higher relative uptake rate and its superiority increased with increasing temperature and photoperiod. S. minutulus conformed to the Droop relationship but N. acicularis did not. A model with a temperature-dependent normalised half-saturation coefficient adequately described the factor interactions of both species. The temperature dependence of the quota model reflected each species' specific adaptation to its ecological niche. The results demonstrate that increases in temperature or photoperiod can partially compensate for a decrease in P-quota under moderately limiting conditions, like during spring in temperate lakes. Thus warming may counteract de-eutrophication to some degree and a relative shift in growth factors can influence the phytoplankton species composition.

To identify the seasonal pattern of nitrogen (N) and phosphorus (P) limitation of phytoplankton in four different lakes, biweekly experiments were conducted from the end of March to September 2011. Lake water samples were enriched with N, P or both nutrients and incubated under two different light intensities. Chlorophyll a fluorescence (Chla) was measured and a model selection procedure was used to assign bioassay outcomes to different limitation categories. N and P were both limiting at some point. For the shallow lakes there was a trend from P limitation in spring to N or light limitation later in the year, while the deep lake remained predominantly P limited. To determine the ability of in-lake N:P ratios to predict the relative strength of N vs. P limitation, three separate regression models were fit with the log-transformed ratio of Chla of the P and N treatments (Response ratio = RR) as the response variable and those of ambient total phosphorus:total nitrogen (TN:TP), dissolved inorganic nitrogen:soluble reactive phosphorus (DIN:SRP), TN:SRP and DIN:TP mass ratios as predictors. All four N:P ratios had significant positive relationships with RR, such that high N:P ratios were associated with P limitation and low N:P ratios with N limitation. The TN:TP and DIN:TP ratios performed better than the DIN:SRP and TN:SRP in terms of misclassification rate and the DIN:TP ratio had the highest R₂ value. Nitrogen limitation was predictable, frequent and persistent, suggesting that nitrogen reduction could play a role in water quality management. However, there is still uncertainty about the efficacy of N restriction to control populations of N₂ fixing cyanobacteria.

Terrestrial loads of dissolved organic carbon (DOC) have increased in recent years in many north temperate lakes, leading to widespread brownification. This process can profoundly alter the relative strength of planktonic and benthic primary production in lakes due to changes in light and nutrient availability. While most existing in situ studies of brownification focus on gradual decadal trends, short‐term flooding can induce rather sudden natural browning events in lakes.We investigated the recovery of a small, temperate, groundwater‐fed shallow lake from a strong natural flooding‐induced browning and nutrient loading event. We hypothesised that along with a return to pre‐flood lake water levels, DOC concentrations would return to pre‐flood concentrations due to bacterial and photolytic mineralisation and flocculation, while total phosphorus (TP) concentrations would show a delayed and potentially incomplete recovery due to sustaining effects of phytoplankton dominance on nutrient dynamics in the lake. Phytoplankton gross primary production (GPP), which surged in response to higher nutrient concentrations and a compressed mixing depth, was expected to follow the same recovery pattern as TP. Benthic periphyton GPP was expected to recover in tandem with improving light conditions.We measured post‐flood water levels, water chemistry, and summer pelagic and benthic GPP. The lake water level declined gradually, returning to pre‐flood levels within 4 years. As hypothesised, DOC concentrations initially declined rapidly, but remained 1.5‐fold higher (c. 22 mg/L) than pre‐flood values 3 years after peak browning. TP concentrations fluctuated greatly between seasons but remained generally elevated in subsequent years, being roughly double pre‐browning values (c. 130 µg/L) 3 years post‐flood. Measurements taken in 2019, 7 years after the initial browning event, showed both DOC and TP concentrations remaining at these elevated levels.Across the intensive study period 3 years before and after peak browning (2010–2015), TP concentrations were positively correlated to summer phytoplankton biomass, while DOC and TP concentrations were negatively correlated to summer periphyton biomass and GPP. Accordingly, the lake's recovery period exhibited a decline of pelagic GPP and a partial recovery of summer periphyton biomass and production, although a full return to pre‐flood values was not recorded in either case. Whole‐lake areal summertime GPP increased due to browning and remained higher than before the browning event.This study provides a rare examination of differences in response rates between DOC, TP, and autotrophic structure in a shallow, temperate lake following a natural flood‐induced extreme browning event. The available data provide compelling evidence that browning events can produce lasting changes in the water chemistry, trophic status, and productive pathways of a lake, persisting beyond the lake's return to pre‐flood water levels. Such delayed or incomplete recoveries from flooding‐induced browning and nutrient loading may have consequences for regional and long‐term temporal trends in lake biogeochemistry, potentially contributing to the widespread phenomenon of incremental brownification. [ABSTRACT FROM AUTHOR]

Kettle holes are often abundant within agriculturally used moraine landscapes. They are highly enriched with nutrients and considered hotspots of carbon turnover. However, data on their primary productivity remain rare. We analysed two kettle holes typical to Germany with common aquatic plant communities during one year. We hypothesised that gross primary production (GPP) rates would be high compared to other temperate freshwater ecosystems, leading to high sediment deposition. Summer GPP rates (4.5-5.1 g C m day) were higher than those of most temperate freshwater systems, but GPP rates were reduced by 90% in winter. Macrophytes dominated GPP from May to September with emergent macrophytes accounting for half of the GPP. Periphyton contributed to most of the system GPP throughout the rest of the year. Sediment deposition rates were high and correlated with GPP in one kettle hole. In contrast, due to prolonged periods of anoxia, aerobic sediment mineralisation was low while sediment phosphorus release was significant. Our results suggest that kettle holes have a high potential for carbon burial, provided they do not fully dry up during warm years. Due to their unique features, they should not be automatically grouped with ponds and shallow lakes in global carbon budget estimates. [ABSTRACT FROM AUTHOR]

1. Although macrophytes play a key role in the structure and functioning of lowland rivers, most of the basic plant, hydrodynamic and sediment-water interactions have only been described qualitatively. We therefore studied quantitatively, the seasonal dynamics of matter deposition and mobilisation inside and outside (free path) a representative patch of arrowhead, Sagittaria sagittifolia, in the lowland River Spree, NE Germany, in August 2006. Our in situ study combined resuspension experiments, a hydrodynamically calibrated erosion chamber and concurrent measurements of the prevailing flow characteristics and bed load. 2. Increasing entrainment rates ( E) of particles ( ESPM) and total P ( ETP), with increments of shear velocity ( U*) from 0.53 to 2.42 cm s−1, were significantly higher inside the plant patch than outside. Indeed, ESPM and ETP at the lowest U* were 8- and 12-fold higher inside than outside the patch, reflecting the resuspension potential of the upper nutrient-enriched layer and the extent of pulsed P inputs even at small increases in U*. 3. Vertical distribution of velocity ( u) revealed a flow pattern of a mixing layer inside the S. sagittifolia patch, and that of a boundary layer in the free path. The highest gradient of u in the mixing layer was located in the water column at about 0.5 m depth, whereas the highest gradient of u for the boundary layer was found near the riverbed. The maximum of U* (1.65 cm s−1) was only 4 mm above the sediment. 4. A plant mosaic provides a low-energetic environment promoting extensive particle trapping and the accumulation of a fine-grained, nutrient-enriched sediment, and forming a large resuspension potential. Consequently, after plant decay and the concomitant increase of U* this material is preferentially entrained at higher rates. Hence, the key role of submerged macrophytes in lowland rivers is more directly related to modifying the dynamic equilibria between vegetation trapping and resuspension, than to the retention of nutrients, particularly P, and the reduction of P loads downstream to other waters. [ABSTRACT FROM AUTHOR]

• High-frequency in-situ SRP results were similar to laboratory results. • SRP concentration was neither affected by filtration nor filter pore size. • In-situ SRP analyzer revealed the pulsed release of SRP in a shallow lake. High-frequency in-situ monitoring of nutrients in lakes is increasingly becoming relevant to understand event-driven nutrient pulses better. These pulses are essential for phytoplankton development but are usually not captured during conventional sampling programs. In-situ monitoring of nutrients in lakes has been lagging despite the role of nutrients in eutrophication. We evaluated an in-situ wet chemistry soluble reactive phosphorus (SRP) analyzer to establish a methodological basis for high-frequency measurements of SRP. The device (based on the molybdate blue method) was evaluated in laboratory and field tests by comparing it with conventional laboratory analysis. The results of the in-situ analyser during laboratory were similar to the conventional analysis (R2 > 0.995, NSE = 0.989, PBIAS = 8.4 %). Repeated measurements by the in-situ analyzer in a 0.1 mg L-1 test solution had a mean and standard deviation of 0.1005 ± 0.0047 and a MAPE of 3.39, indicating the stability of the measurements. During field tests in a hypertrophic lake with anoxic hypolimnion, the in-situ analyzer SRP concentrations were lower and divergent from the laboratory analysis; due to potential interference of the in-situ analyzer's reagents by sulfides. Under aerobic conditions during field tests, the in-situ P results of a eutrophic, mixed shallow lake were similar to those from water samples analyzed in the laboratory (R2 = 0.562 – 0.812, NSE = 0.397 – 0.707, PBIAS = -8.5 – 24.6 %). There was no difference in SRP concentration among the unfiltered, 0.2 µm, and 10 µm pore size filtered samples; filter pore size did not play a role in the measured SRP concentrations. The in-situ SRP analyzer revealed previously unobserved phenomena, e.g., multiple strongly discontinuous SRP peaks reaching almost 0.1 mg L-1; it therefore, offers a solid methodological basis for high-frequency monitoring of SRP. Incorporating high-frequency in-situ nutrient analysis in lake monitoring provides insights into lakes' short-term nutrient dynamics. Such information is essential for understanding the effects of extreme events like droughts, storms, and heat waves. [ABSTRACT FROM AUTHOR]