Oxygen consumption in seasonally stratified lakes decreases only below a marginal phosphorus threshold

Abstract

Areal oxygen (O2) consumption in deeper layers of stratified lakes and reservoirs depends on the amount of settling organic matter. As phosphorus (P) limits primary production in most lakes, protective and remediation efforts often seek to reduce P input. However, lower P concentrations do not always lead to lower O2 consumption rates. This study used a large hydrochemical dataset to show that hypolimnetic O2 consumption rates in seasonally stratified European lakes remain consistently elevated within a narrow range (1.06 ± 0.08 g O2 m−2 d−1) as long as areal P supply (APS) exceeded 0.54 ± 0.06 g P m−2 during the productive season. APS consists of the sum of total P present in the productive top 15 m of the water column after winter mixing plus the load of total dissolved P imported during the stratified season, normalized to the lake area. Only when APS sank below this threshold, the areal hypolimnetic mineralization rate (AHM) decreased in proportion to APS. Sediment trap material showed increasing carbon:phosphorus (C:P) ratios in settling particulate matter when APS declined. This suggests that a decreasing P load results in lower P concentration but not necessarily in lower AHM rates because the phytoplankton community is able to maintain maximum biomass production by counteracting the decreasing P supply by a more efficient P utilization. In other words, in-lake organic matter production depends only on APS if the latter falls below the threshold of 0.54 g P m−2 and correspondingly, the atomic C:P ratio of the settling material exceeds ~155.

Hypolimnetic oxygen depletion rates in deep lakes: Effects of trophic state and organic matter accumulation

“…Below a TP concentration of ~ 10 mg P m −3 , WCM covaries closely with TP concentration. This is in line with the observation of Müller et al (2019) that AHM in seasonally stratified lakes increases approximately in proportion to the areal amount of bio‐available P but levels off above this threshold value. O 2 consumption in lakes > 100 m deep may immediately respond to a decrease of TP for concentrations below this threshold.…”

Nitrogen removal processes in lakes of different trophic states from on-site measurements and historic data

“…Phytoplankton is dominated by the cyanobacterium Planktothrix rubescens . Over the past few decades, the lake has undergone a remediation phase, which has curtailed water column total P concentrations from 520 mg P m −3 in the 1970s to ~ 24 mg P m −3 today (Müller et al 2019 ). Moreover, peak N was observed in the 1980s with up to 2.3 g N m −3 (Wehrli et al 1997 ).…”

“…This can be excluded in Lake Baldegg with its high concentration of NO 3 − that was never exhausted in the productive zone, because the lake was fully productive since more than a century (Lotter 1998) with P concentration not limiting production. Indicative for the situation is the hypolimnetic O 2 consumption that is unchanged since measurements begun in 1983 even though TP concentration decreased from 520 to 25 mg m −3 (Müller et al 2019), and net ecosystem production of ~ 90 g C m −2 year −1 both in 1995/1996 (Müller et al 2012) and 2013(Steinsberger et al 2017 determined from sediment traps. Hence, the good correlation between NRR and load requires that a large fraction of the imported N is removed within the epilimnion, e.g.…”

Effects of phosphorus control on primary productivity and deep-water oxygenation: insights from Lake Lugano (Switzerland and Italy)

“…The main processes that cause AHOD in lakes are (1) the mineralisation and respiration of PPR settling in the water column, (2) the further processing of PPR recently deposited at the sediment's surface and (3) the oxidation of reduced substances (e.g. ammonium, hydrogen sulphide and methane) released from the sediments (Müller et al, 2019). A strong coupling between PPR and AHOD should arise where/when processes (1) and (2) are predominant, whereas process (3) could weaken the PPR-AHOD link.…”

“…1) to investigate the effect of phosphorus reduction on hypolimnetic oxygen. We hypothesised that this effect was weak because (1) primary productivity is relatively unaffected by phosphorus at high-to-moderate concentrations (Anneville & Pelletier, 2000;Müller et al, 2019) and (2) the transmission of effects through the phosphorus-hypolimnetic oxygenation pathway is dampened by external variation (i.e. by variables external to trophic state; Rogora et al, 2018).…”