Lake toxin concentrations increase when phosphorus is reduced

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Lake toxin concentrations increase when phosphorus is reduced

https://doi.org/10.21203/rs.3.rs-4328909/v1

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Harmful algal blooms are an urgent global societal problem threatening safe use of lakes for drinking and recreation, whereas the management solutions are hotly debated. Reducing phosphorous loading can be successful in decreasing cyanobacterial biomass but the effect on toxin concentration is unclear. The underlying mechanisms, laboratory data, limited circumstantial evidence from field observations and models suggest toxin concentrations may actually increase. What has been missing is field data to support or refute this. We analyzed timeseries data from hundreds of lakes (US National Lakes Assessment) and show that when phosphorus only is reduced, microcystin (the toxin produced by the cyanobacteria Microcystis) concentration increases. However, when nitrogen is reduced together with phosphorus, microcystin concentration decreases along with biomass.

Earth and environmental sciences/Limnology

Biological sciences/Ecology/Freshwater ecology

Earth and environmental sciences/Biogeochemistry/Element cycles

Earth and environmental sciences/Natural hazards

Cyanobacterial harmful algae blooms (CyanoHABs), most prominently those of Microcystis and the produced hepatotoxin microcystin (MC), threaten safe use of freshwater resources for drinking and recreation as well as ecosystem health ^1,2. A prominent example for this is the interruption of the drinking water supply of Toledo in 2014 ³. The National Lakes Assessment (NLA) revealed the presence of HABs in 45% and of MCs in 21% of all examined US lakes in 2017 ⁴. The problem is predicted to get worse in a future warmer climate ^5,6.

Effective management of CyanoHABs is a grand societal challenge. The most common strategy is to control cyanobacteria biomass by reducing growth-limiting nutrients ¹. A debate over which nutrient(s) to reduce, nitrogen and/or phosphorus, is ongoing in the limnological literature ^7,8, but a phosphorus only reduction is the more commonly applied approach in practice. There are many success stories where phosphorus reduction has led to a decrease of phytoplankton biomass (Chlorophyll a concentration - Chla) ⁸. But what about toxin concentrations, presumably one of the most important targets of a management action? The implicit, and entirely reasonable assumption (although faulty as we will show here), is that toxin concentrations will decrease proportionally with biomass. Thus, it is assumed that phytoplankton stoichiometry (here MC/Chla) remains unchanged. This idea has deep roots in limnology and oceanography ever since Redfield´s seminal work illustrating relatively constant nitrogen to phosphorus ratio in marine phytoplankton ⁹, which was subsequently extended to include other elements (e.g. C₁₂₄N₁₆P₁S_1.3K_1.7Mg_0.56Ca_0.5Fe_0.0075Zn _0.0008Cu _0.00038Cd _0.00021Co _0.00019) ^10,11.

Natural phytoplankton communities consist of numerous species and strains including those that can and cannot produce toxins ^1,2,12. Further, the toxigenic strains do not constantly synthetize toxins (i.e. the MC cell quota is variable) ¹². Reducing phosphorus loading will lower phytoplankton biomass, but also increase the availability of other resources, like nitrogen and light, which will in turn change the MC quota and community composition.

For MC production, nitrogen and light are important resources: nitrogen is part of the MC molecule (min. 7 N/MC) and light is needed as an energy source for its synthesis². This dependence is evident in a number of laboratory experiments, which show increasing nitrogen results in higher MC quota (fig. 1A, for method description see SI). For example, Van de Waal (V09 in fig. 1A) ¹³ found that the cellular toxin quota in nitrogen replete medium (containing 12 mM NO₃) was more than double (log2 of ratio of MC quotas is 1.1) compared to nitrogen deplete medium (0.02 mM NO₃). A clear outlier is the study by Pimentel and Giani (PG14 in fig. 1A) ¹⁴, which showed increased mcyD expression and MC production under low nitrogen cultivation. The authors suggest that severe nitrogen-limitation caused oxidative stress, which reduced MC production (see discussion about biological role below). However, the majority of studies do not show this effect. Other studies investigated the effect of irradiance on MC quota (fig. 1B). Most results show higher light intensity increases MC quota. For instance, the fold-change of MC quota was 1.2 (e.g. MC quota has more than doubled) when cultivated at 80 compared to 20 µmol photons/m²/s (UG92 in fig. 1B) ¹⁵. However, a few studies showed a decreased MC quota with higher irradiance. For example, Phelan and Downing (PD11 in fig. 1B) ¹⁶ used continuous light intensities of 8 and 37 µmol photons/m²/s and showed a reduction of MC quota to 22% on average (log2 of ratio = - 2.1). They suggest that missing dark phases in their experimental set-up prevent sufficient regeneration of cell damages and respective MC synthesis. Hence, this observation is not relevant to the field. Generally, there is a lot of variability which can be attributed to strain and experimental variability, as well as feedbacks. For example, higher nitrogen in nitrogen-limited cultures will increase biomass and decrease light due to self-shading, which in turn may decrease MC production. Nonetheless, laboratory data suggest that increased availability of nitrogen and light, an expected result of a phosphorus only reduction, increase MC production and quota. A phosphorus only reduction means the toxigenic strain will become more toxic.

A change in MC quota (of the toxigenic strain) may also translate to a change in community composition. This relates to the biological role of MC, which is not yet fully understood. Potential functions include protection against oxidative stress ^18,19, carbon limitation ^20,21 or parasite infection ^22,23. Despite this uncertainty, the highly conserved genome region of the enzyme complex for synthesis ²⁴ and wide distribution of cyanotoxins ²⁵ indicate a substantial fitness benefit for the producer. Hence, higher MC quota (due to increased nitrogen and light availability) will benefit the toxin-producing cells in competition against other phytoplankton and result in an increased fraction in the community, which is also evident in field observations. Welker et al. ²⁶ investigated Microcystis biomass and MC dynamics in Lake Müggelsee and concluded that toxin-producing strains dominate at times of high nitrogen and low phosphorus availability. Also, several other studies show that the growth of toxigenic strains was associated with nitrogen availability in lakes ^17,27,28. For instance, in Lake Mikata in 2005 ²⁷, the population becomes progressively more toxigenic as phosphorus decreases and nitrogen increases. Hence, a phosphorus only reduction means there will be more of the toxigenic cells.

In summary, the underlying mechanisms, laboratory experiments and limited indirect evidence from field observations, i.e. none of the above analyses include direct measurements of how MC concentration changes with total phosphorus (TP), suggest that higher availability of nitrogen and light increases both, MC cell quota and toxigenic fraction. This may have important ramifications for lake management. For a phosphorus only reduction, the phytoplankton stoichiometry (here MC/Chla) will change in a direction that will counteract the reduction in biomass. Decreasing phosphorus may actually increase toxin concentration. This was predicted by a model that includes these mechanisms, which estimated that a 40 % phosphorus only reduction will increase MC concentration by 14 % in Lake Erie ¹⁷.

Several management case studies of phosphorus reduction in lakes showed reduced biomass ⁸ but whether toxin concentrations decreased proportionally is often not quantified. We explored this question for the lakes summarized by Schindler et al. by performing a strategic literature review (for methodology see SI). Of the 35 lakes, time-series data of MC concentrations are only available for 9 lakes, and they do not cover the time span from before to after the phosphorus only reduction. Therefore, there is a lack of direct field observations that show how toxin concentrations change when total phosphorus is reduced.

Alfred Redfield analyzed large oceanic datasets to quantify stoichiometric relations in phytoplankton. In a similar manner and methodology, we seek insight into the relation between total phosphorus, biomass and MC concentration in field observations. Specifically, we ask: Do decreasing phosphorus concentrations in lakes result in an increase of MC concentration?

We analyzed the NLA dataset, which includes relevant observations, including TP, total nitrogen (TN), Chla, dissolved inorganic nitrogen (DIN, i.e. biological available nitrogen), turbidity (NTU, i.e. parameter for available light) and MC concentration (for methodology see SI). Several previous studies analyzed NLA data, using different approaches to find relations of MCs and environmental variables across lakes ^29–32. For example, Merder et al. ²⁹ used the dataset to explore the impact of warming on the likelihood of high MC concentration, using a framework of Generalized Additive Models for Location, Scale and Shape (GAMLSS) including parameters previously discovered to affect MCs. Whereas those studies use relatively complex models including flexible regressions and smoothing functions, we used a much simpler and more targeted approach of basic regression analyses. Further, rather than looking at correlations across lakes, we limit our analysis to the change of MC concentrations within individual lakes across time, because the absolute concentrations may vary among different lakes due to other factors, and this is more representative of a change due to management.

Samples were collected in 2007, 2012 and 2017 and the database comprises 9821 sampling events including 1999 detections of MC concentrations (fig. 2). This included 453 time pairs, i.e. data points with detectable MC concentrations at different times within a lake. We correlate changes of different parameters (e.g. MC concentrations) with changes in TP concentrations, where change is quantified as relative change using log2 of the ratio, i.e. fold change. However, note that the observed change in phosphorus may not be due to management, but natural hydrological variations. This means that nitrogen may also change along with it (e.g. dry or wet years affect nutrient nonpoint source input and flushing). Thus, a change in TP may or may not be associated with a concomitant change in TN so it may be a representative of a phosphorus only or dual nutrient reduction of a management plan. We therefore split the dataset into two groups (see visualization in fig. 2): in the first group TP and TN concentrations change in the same direction, corresponding to a dual nutrient reduction. In the second group TP and TN concentrations change in opposite directions, corresponding to a phosphorus only reduction. This group was further divided into two sub-groups which are either phosphorus limited or not, depending on changes in biomass in the same or opposite direction as TP concentrations. We first present and discuss the results for the time pairs where phosphorus only was reduced and which were phosphorus limited.

In fig. 3 changes in TP concentrations are shown, correlated with changes of various other parameters. A phosphorus only management action would move a lake from right to left on the x- axis. The phosphorus limited subset was defined based on a positive correlation between changes in TP and biomass (fig. 3 A). Changes in TP correlate negatively with changes in DIN concentration (fig. 3 B1) and positively with changes in turbidity (fig. 3 B2), showing an increasing availability of nitrogen and light with reduced TP concentrations. A negative correlation of TP concentrations with MC concentration per biomass was identified (fig. 3 C). This analysis illustrates that if TP concentration decreases, the phytoplankton stoichiometry changes. Specifically, MC concentrations per biomass increase, consistent with the above mechanism: a reduction in biomass results in higher availability of nitrogen and light (both observed in the data), which stimulates MC production and increases toxigenic fraction. This counteracts the decrease in biomass (theoretical dashed line in fig. 3 D) and results in an increase in MC concentrations with reduced TP concentrations (solid line in fig. 3 D).

The results for different subsets of the data provide further insights into how MC concentration is expected to change under different management scenarios. In figure 4, we show the correlations of TP and MC concentrations (solid lines) and the corresponding theoretical lines assuming a constant phytoplankton stoichiometry (i.e. changes in biomass, dashed lines) for the different groups defined in fig. 2. In the subset where TP and TN concentrations change concomitantly, corresponding to a dual nutrient reduction management strategy, MC concentration decreases along with biomass (figure 4B). This is consistent with the mechanisms discussed above: nitrogen availability decreased and less MC is produced (see also fig. S2). In the group corresponding to a phosphorus only reduction, lower TP resulted in an increase in MC concentration (fig. 4 C). Biomass does not change with reduced TP concentrations (dashed line), because not all lakes were phosphorus limited. This prompted us to split the group into sub-groups being phosphorus limited and not phosphorus limited. In the group of phosphorus limited lakes, MC concentrations increase with reduced TP concentrations as discussed above (fig. 4 D). In the group which are not phosphorus limited, a reduction in TP is associated with an increase in TN and an increase in biomass, and MC concentration changes along with biomass (fig. 4 E and S4).

The NLA data are consistent with the underlying mechanisms, laboratory observations, limited circumstantial evidence from field observations, field data and models, and strengthen the notion that reducing only phosphorus may reduce biomass, but toxin concentrations will not reduce proportionally. Hence, reducing only phosphorus loading to a lake is risky, and field data show that MC concentrations actually increase. A dual reduction of phosphorus and nitrogen is more effective in reducing toxin concentrations.

The “Redfield ratio” of a constant nitrogen to phosphorus ratio has served as a useful reference for data analysis and modeling in limnology and oceanography for almost a century. At the same time, it is clear that phytoplankton stoichiometry does change and that this can be important to consider in some cases ^11,33. Our findings here show that, when it comes to toxin content, specifically MC, it is important to consider variable stoichiometry, and highlight the importance of moving on from the paradigm that “reducing phosphorus equals reducing biomass equals reducing toxicity” ³⁴. This will require more field observations and toxin quantification should be included as standard measure when evaluating lake status before and after management action.

Acknowledgement

This project was financed by Deutsche Forschungsgemeinschaft (DFG), GRK 2032: Urban Water Interfaces (UWI) and National Oceanographic and Atmospheric Administration grant NA18NOS478017. Thanks to Falk Eigemann, Jutta Hoffmann, Carsten Lackner and Behnam Zamani for constructive feedback.

Author contributions

Conceptualization: F.L.H. and C.S., Data curation: C.S., T.N. and J.P., Formal analysis: C.S., Funding acquisition: F.L.H., Investigation: C.S. and T.N., Methodology: F.L.H. and C.S., Project administration: F.L.H., Resources: F.L.H., Supervision: F.L.H., Validation: C.S., Visualization: C.S. and F.L.H. Writing - original draft: C.S. and F.L.H., Re-writing draft: all authors

Competing interest declaration

The authors declare no competing interest.

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Lake toxin concentrations increase when phosphorus is reduced

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Harmful algal blooms and their management

SHOULD toxin concentration increase when phosphorus loading is reduced?

DO toxin concentrations increase when phosphorus loading is reduced?

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