Hydrobiologia

Paerl, Hans W.; Havens, Karl E.; Xu, Hai; Zhu, Guangwei; McCarthy, Mark J.; Newell, Silvia E.; Scott, J. Thad; Hall, Nathan S.; Otten, Timothy G.; Qin, Boqiang

doi: 10.1007/s10750-019-04087-ypmid: N/A

Cyanobacterial harmful algal blooms (CyanoHABs) are an increasingly common feature of large, eutrophic lakes. Non-N2-fixing CyanoHABs (e.g., Microcystis) appear to be proliferating relative to N2-fixing CyanoHABs in systems receiving increasing nutrient loads. This shift reflects increasing external nitrogen (N) inputs, and a > 50-year legacy of excessive phosphorus (P) and N loading. Phosphorus is effectively retained in legacy-impacted systems, while N may be retained or lost to the atmosphere in gaseous forms (e.g., N2, NH3, N2O). Biological control on N inputs versus outputs, or the balance between N2 fixation versus denitrification, favors the latter, especially in lakes undergoing accelerating eutrophication, although denitrification removal efficiency is inhibited by increasing external N loads. Phytoplankton in eutrophic lakes have become more responsive to N inputs relative to P, despite sustained increases in N loading. From a nutrient management perspective, this suggests a need to change the freshwater nutrient limitation and input reduction paradigms; a shift from an exclusive focus on P limitation to a dual N and P co-limitation and management strategy. The recent proliferation of toxic non-N2-fixing CyanoHABs, and ever-increasing N and P legacy stores, argues for such a strategy if we are to mitigate eutrophication and CyanoHAB expansion globally.

Kuha, Jonna; Järvinen, Marko; Salmi, Pauliina; Karjalainen, Juha

doi: 10.1007/s10750-019-04086-zpmid: N/A

Organic matter (OM) other than living phytoplankton is known to affect fluorometric in situ assessments of chlorophyll in lakes. For this reason, calibrating fluorometric measurements for OM error is important. In this study, chlorophyll (Chl) fluorescence was measured in situ in multiple Finnish lakes using two sondes equipped with Chl fluorometers (ex.470/em.650–700 nm). OM absorbance (A420) was measured from water samples, and one of the two sondes was also equipped with in situ fluorometer for OM (ex.350/em.430 nm). The sonde with Chl and OM fluorometers was also deployed continuously on an automated water quality monitoring station on Lake Konnevesi. For data from multiple lakes, inclusion of water colour estimates into the calibration model improved the predictability of Chl assessments markedly. When OM absorbance or in situ OM fluorescence was used in the calibration model, predictability between the in situ Chl and laboratory Chl a assessments was also enhanced. However, correction was not superior to the one done with the water colour estimate. Our results demonstrated that correction with water colour assessments or in situ measurements of OM fluorescence offers practical means to overcome the variation due to OM when assessing Chl in humic lakes in situ.

Vuorio, Kristiina; Järvinen, Marko; Kotamäki, Niina

doi: 10.1007/s10750-019-04161-5pmid: N/A

Cyanobacteria may limit recreational use of waters and have negative impacts on ecosystem services. The aim of this study was to determine phosphorus (P) thresholds for cyanobacterial taxa, which form toxin-producing blooms (mass occurrences) in boreal lakes. These thresholds help to set robust P reduction targets for restoration measures. We used long-term (> 40 years) June–August data from > 2000 Finnish lakes to evaluate the total P (TP) thresholds for the most important bloom-forming cyanobacterial genera and selected Microcystis and Dolichospermum species. The analyses were carried out separately for oligohumic, mesohumic and polyhumic lakes. TP threshold values varied between 10 and 61 µg l−1. The values were lower in oligohumic (water colour < 30 mg Pt l−1) than mesohumic (water colour 30–90 mg Pt l−1) lakes. The highest TP threshold (50 µg l−1) was observed for Microcystis in polyhumic lakes, and the lowest (10 µg l−1) for Planktothrix in oligohumic lakes.

Nürnberg, Gertrud K.

doi: 10.1007/s10750-019-04094-zpmid: N/A

The withdrawal of hypolimnetic water enriched with nutrients and reduced substances has been used as a lake restoration technique (hypolimnetic withdrawal, HW) for more than 60 years. By reducing internal phosphorus loading, HW treats the cause of much of the water-quality deterioration in eutrophic stratified lakes, including cyanobacteria blooms. To support future applications, a feasibility study is presented that determines the applicability of HW to a drinking water source lake for the City of Stockholm as an example. Necessary treatment of the HW water includes a technically advanced facility. The possible application of passive siphoning by gravity limits recurring energy costs. The most efficient performance is ensured by the monitoring of HW operational variables and water quality in the source and receiving water. A review of the scientific literature and the worldwide web confirms that new and continued HW applications improve water quality in stratified lakes with anoxic hypolimnia. A previously developed model for the prediction of epilimnetic TP decreases from the long-term TP export via HW was supported by results from several new applications. Benefits of flow regulation considering climate change variability have been reported, suggesting that HW could play a progressively important role in lake management.

Niemistö, Juha; Silvonen, Soila; Horppila, Jukka

doi: 10.1007/s10750-019-04160-6pmid: N/A

Effects of hypolimnetic aeration (pumping of epilimnetic water into the hypolimnion) on the quantity of settling material in eutrophied Lake Vesijärvi, Finland were studied by comparing spatially comprehensive gross sedimentation rates as dry and organic matter prior to aeration activity and during two aerated years. Possible changes in the organic matter (as loss on ignition, LOI), carbon (C) and nitrogen (N) contents and changes in the C/N ratio of the settling material and surface sediment were quantified. Thermal stratification broke up earlier due to aeration and was followed by sedimentation peaks. The absolute amount of dry and organic matter as well as C and N settling to the lake bottom were significantly higher in the aerated years. Increased sedimentation rates were especially pronounced in the deep zones indicating enhanced sediment focusing. Increased sedimentation of C and N reflected higher primary production during the aerated years, which most likely was associated with increased temperature and turbulence and the subsequent regeneration and recycling of nutrients in the water body. Aeration seemed to slightly enhance degradation, but contrary to its ultimate aim, it failed to decrease the phosphorus content of the water column and deposits of organic material in the deep zones of the lake.