1 INTRODUCTION

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.

One potential factor limiting benthic GPP is the supply of inorganic carbon, in particular carbon dioxide (CO₂). CO₂ limitation of GPP has traditionally been regarded as uncommon since atmospheric CO₂ uptake was considered sufficient to support pelagic GPP (Schindler et al., 1972), but recent studies show that high CO₂ concentrations can stimulate pelagic GPP (Hein, 1997; Jansson et al., 2012; Kragh & Sand-Jensen, 2018) and whole-lake GPP (Hamdan et al., 2018), especially in nutrient-rich waters (Hammer et al., 2019; Spijkerman, 2010; Verspagen et al., 2014). In nutrient-poor boreal lakes, however, a positive effect of CO₂ on pelagic GPP is not necessarily observed (Hessen et al., 2017). In benthic habitats, CO₂ limitation might be more pronounced than in the pelagic habitat due to strong boundary layer effects at the water sediment interface that constrain diffusion of CO₂ from the water to the benthic algae (Riber & Wetzel, 1987). However, processes within the sediment (such as methanogenesis) might also provide benthic algae with CO₂ (Gruca-Rokosz & Tomaszek, 2015) although diffusion from such processes should also be constrained by a lack of turbulence. While benthic microalgae in shallow marine systems often show a positive response to CO₂ (Johnson et al., 2013; Vieira et al., 2016), the CO₂ effect on soft-bottom benthic GPP in freshwater systems is largely unknown (Brown et al., 2019). Hence, the extent to which CO₂ availability limits aquatic GPP in different types of ecosystems is complex, which indeed impairs the understanding of factors controlling whole-lake GPP.

The dissolved inorganic carbon (DIC) concentration in water is controlled by a range of processes, including input from the catchment, exchange with the atmosphere, in situ GPP, respiration, and photooxidation (Vonk, 2015). Furthermore, input of allochthonous dissolved organic carbon (DOC) often causes lakes to be CO₂-supersaturated (Cole et al., 1994) via increased heterotrophic respiration and photooxidation (Vachon et al., 2017). Hence, allochthonous supply of both DIC and DOC can be important sources of CO₂ in lakes. Allochthonous DOC is often coloured, and thereby reduces light penetration through the water column (Ask et al., 2009). Thus, it is likely that allochthonous DOC, by increasing the CO₂ supply, could indirectly stimulate GPP at low to moderate DOC concentrations where light is sufficient for photosynthesis, but not at high concentrations where light limitation will be pronounced.

In this study, we tested for CO₂ limitation of benthic microalgal GPP in a freshwater system. We collected lake surface sediment (with associated benthic microalgae) and incubated these at different light levels after addition of a direct (DIC) or indirect (CO₂ production via mineralisation of DOC) source of CO₂. We hypothesised that: (1) benthic GPP is CO₂ limited; (2) CO₂ limitation of benthic GPP can be relieved both by a direct and an indirect supply of CO₂; and (3) the response in benthic GPP to CO₂ addition is dependent on light availability.

中文翻译：

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北方湖泊底栖初级生产的二氧化碳限制

1 简介

生长在软沉积物上的底栖微藻（此处为底栖微藻）的初级生产总值（GPP）可能主导整个生态系统的 GPP（Ask 等人，  2009 年；Vadeboncoeur 等人，  2001 年）和更高营养水平的能源供应（Karlsson 和 Byström） ,  2005 年）。由于底栖微藻可以获取沉积物中的养分（Bonilla 等人，  2005；Daniels 等人，  2015 年），它们被认为主要受光照的控制（Ask 等人，  2009 年；Hamdan 等人，  2021 年；Hansson） ,  1992）。然而，针对底栖系统中 GPP 的实验研究远少于中上层系统，我们对湖泊中底栖 GPP 局限性的认识仍存在很大差距。

限制底栖 GPP 的一个潜在因素是无机碳的供应，特别是二氧化碳 (CO ₂ )。GPP 的CO ₂限制传统上被认为是不常见的，因为大气 CO ₂吸收被认为足以支持远洋 GPP（Schindler 等，  1972），但最近的研究表明，高 CO ₂浓度可以刺激远洋 GPP（Hein，  1997； Jansson 等人，  2012 年；Kragh & Sand-Jensen，  2018 年）和全湖 GPP（Hamdan 等人，  2018 年），尤其是在营养丰富的水域（Hammer 等人，  2019 年；Spijkerman，  2010 年；Verspagen 等人） ., 2014 年）。然而，在营养贫乏的北方湖泊中，不一定观察到 CO _{2对远洋 GPP 的积极影响（Hessen 等人，} 2017 年）。在底栖生境中，CO ₂限制可能比在远洋生境中更明显，因为水沉积物界面处的强边界层效应限制了 CO ₂从水中向底栖藻类的扩散（Riber & Wetzel，  1987 年）。然而，沉积物中的过程（如产甲烷作用）也可能为底栖藻类提供 CO ₂ (Gruca-Rokosz & Tomaszek,  2015) 尽管来自这些过程的扩散也应该受到缺乏湍流的限制。虽然浅海系统中的底栖微藻通常对 CO ₂表现出积极的反应（Johnson 等人，  2013 年；Vieira 等人，  2016 年），但 CO ₂对淡水系统中软底底栖 GPP 的影响在很大程度上是未知的（Brown 等人）等人，  2019 年）。因此，CO ₂可用性限制不同类型生态系统中水生 GPP 的程度是复杂的，这确实损害了对控制全湖 GPP 的因素的理解。

水中溶解的无机碳 (DIC) 浓度受一系列过程控制，包括集水区的输入、与大气的交换、原位 GPP、呼吸作用和光氧化（Vonk，  2015 年）。此外，外源溶解有机碳 (DOC) 的输入通常会通过增加异养呼吸和光氧化作用（Vachon 等人，  2017 年）导致湖泊成为 CO ₂过饱和（Cole 等人，  1994年）。因此，DIC 和 DOC 的异地供应可能是湖泊中 CO _{2的重要来源。}异地 DOC 通常是有色的，从而减少了通过水柱的光穿透（Ask et al.,  2009）。因此，通过增加 CO ₂供应，外源 DOC 可能会在光足以进行光合作用的低至中等 DOC 浓度下间接刺激 GPP，但在光限制明显的高浓度下则不会。

在这项研究中，我们测试了淡水系统中底栖微藻 GPP 的 CO ₂限制。我们收集了湖表沉积物（与相关的底栖微藻），并在添加直接（DIC）或间接（通过 DOC 矿化产生 CO 2 ）源 CO _2后在不同光照水平下培养这些沉积物。我们假设： (1) 底栖 GPP 受 CO ₂限制；(2)可以通过直接和间接供应 CO _{2来缓解底栖 GPP 对 CO 2}的限制；(3) 底栖 GPP 对 CO ₂添加的响应取决于光的可用性。