An experimental test of climate change effects in northern lakes: Increasing allochthonous organic matter and warming alters autumn primary production

Abstract

Climate changes are predicted to influence gross primary production (GPP) of lakes directly through warming and indirectly through increased loads of allochthonous coloured dissolved organic matter (cDOM) from surrounding landscapes. However, few studies have investigated this combined effect. Here we tested the effects of warming (elevated 3℃) and cDOM input (three levels of humic river water addition) on GPP in autumn (2 months including open water and ice‐covered periods) in experimental pond ecosystems. The cDOM input decreased whole‐ecosystem GPP at natural temperature conditions mainly as a result of lower benthic GPP not fully counteracted by an increase in pelagic GPP, while warming increased whole‐ecosystem GPP due to a positive response of mainly pelagic GPP at all levels of cDOM input. Warming delayed autumn ice cover formation by 2 weeks but did not affect light availability in the water column compared to ambient ice‐covered treatments. Gross primary production during this period was still affected by warming and cDOM. The results stress the importance of accounting for multiple climate drivers and habitats when predicting lake GPP responses to climate change. We conclude that climate change may shift whole‐ecosystem GPP through different responses of habitat‐specific GPP to increasing cDOM inputs and warming.

The importance of allochthonous organic matter quality when investigating pulse disturbance events in freshwater lakes: a mesocosm experiment

“…On the other hand, the same events could enhance eutrophication in water bodies which are already nutrient sufficient, as reported elsewhere (Gao et al, 2021). In contrast, other types of allo-OM sources, which are more similar to HuminFeedÒ characteristics, such as those associated with humic waters (Hamdan et al, 2021), could lead to higher nutrient constraints (i.e. high C relative to N and P) at all trophic levels.…”

“…Furthermore, a commercially available humic substance called HuminFeedÒ (Humintech, Germany) has been widely used in mesocosm experiments investigating allo-OM inputs to lakes (Rasconi et al, 2015;Urrutia-Cordero et al, 2017;Lebret et al, 2018;Minguez et al, 2020). Considering the complexity of catchment-dependent allo-OM characteristics, other mesocosm studies have mimicked allo-OM using a range of different types of natural mixtures such as i) humic soil and inorganic N and P additions (Lefe ´bure et al, 2013), ii) leaf leachate and inorganic N and P additions (Hitchcock et al, 2016;Brighenti et al, 2018;Tonetta et al, 2018), iii) filtered and concentrated humic lake water (Nicolle et al, 2012;Cooke et al, 2015;Hamdan et al, 2021), and iv) organic matter-rich top soils from inflowing rivers produced by preserving natural fine soil particles (Liess et al, 2015). In addition, in order to separate the browning effect from allo-OM as energy source effects, some authors simulated light reduction by adding a black net to reduce solar irradiance (Urabe et al, 2002;Brighenti et al, 2018).…”

Contrasting impacts of warming and browning on periphyton

“…We created the cDOM gradient by combining continuously provided input of cDOM-rich water, collected biweekly from the naturally cDOM-rich small boreal river Hörneån located 45 km northwest of EXEF, with tap water derived from the Umeå municipality that uses ground water as source (control treatment; Table 1 for water chemistry for tap and river water). The water of each individual pond was warmed by a land-based individual heat exchanger and circulated through a filter cube (10 PPI) in each pond (see Capo et al 2021;Hamdan et al 2021 for specifics). The ambient ponds were subjected to the same circulation process without heat exchangers.…”

“…In addition, nutrients, light, and temperature combined have interactive effects on aquatic algal growth (De Senerpont Domis et al 2014;Endo et al 2017;Burrows et al 2021). For instance, algae become more efficient at utilizing nutrients at higher temperatures, resulting in a synergetic effect on algal growth (Rhee and Gotham 1981;Hamdan et al 2021) and biomass accrual , where the net responses are further constrained by light. Free-floating algae (phytoplankton) are restricted to utilizing nutrients from the water and can position themselves at optimal light conditions, whereas periphytic (attached) algae are stationary and thus restricted to use the light reaching their location.…”

Carbon dioxide limitation of benthic primary production in a boreal lake

“…Gross primary production (GPP) by benthic microalgae growing on soft sediments (here benthic microalgae ) may dominate whole‐ecosystem GPP (Ask et al, 2009; Vadeboncoeur et al, 2001) and energy supply for higher trophic levels (Karlsson & Byström, 2005). As benthic microalgae have access to nutrients in the sediment (Bonilla et al, 2005; Daniels et al, 2015) they are regarded as primarily controlled by light availability (Ask et al, 2009; Hamdan et al, 2021; Hansson, 1992). However, experimental studies focusing on GPP in benthic systems are far less numerous than in pelagic systems, and there are still large gaps in our understanding regarding limitations of benthic GPP in lakes.…”