Biodeposition from four bivalves species using natural estuary water in Australia

The threatened status of shellfish reefs has been well established globally (e.g Beck et al 2011) however the ecological consequences of these losses is still largely unknown. In Australia, shellfish reefs are one of the most imperilled marine habitat types (Gillies et al 2018), due to historical overharvest and widespread eutrophication of coastal waters through the use of fertilizers, livestock and human waste. Marine bivalves are important ecosystem engineers providing habitat, shelter and a food source for other species in benthic soft-sediment environments. In addition, filter-feeding bivalves link benthic and pelagic components of ecosystems through filtration and excretion. Through their filter feeding, they produce large amounts of faeces (digested seston) and pseudofaeces (rejected particles bound up in mucus) which are deposited on the benthos. This process brings energy and nutrients from the pelagic system to the benthic system (bentho-pelagic coupling). The removal of large quantities of seston can serve an important ecosystem function by improving water quality and clarity. The filtration of water performed by bivalves has been demonstrated to reduce water turbidity, improving light penetration and thereby enhancing growing conditions for seagrasses (Wall et al 2008). In systems where healthy populations of bivalves remain, they can filter a volume equivalent or larger than the entire estuary volume within the residence time of the water (zu Ermgassen et al 2013). While such densities of oysters are rare today, this highlights the critical ecosystem services that are lost when oyster reefs decline. Furthermore, it demonstrates the potential functions that can be regained through oyster reef restoration. Given the increasing awareness of the decline of these ecosystems, interest in restoration efforts to restore critical ecosystem functions has been growing. However, conservation and restoration decision making is underpinned by reliable quantification of relevant ecosystem services (zu Ermgassen et al 2016). For example, there are plans to restore some of the natural oyster reefs of Sydney Rock Oyster (Saccostrea glomerata) in Port Stephens, New South Wales. One of the main drivers motivating this restoration project is restoring lost ecosystem services. The filtration rates of Australian oysters has been demonstrated in aquarium studies using filtered water augmented with algae, yet little is known about filtration and biodeposition rates of oysters using raw seawater. In this study, we provide the first evaluation of the filtration and biodeposition rate of four species of bivalves using raw seawater, providing a proxy for natural biodeposition rates. As such, this study provides a first indication of the filtration/nutrient cycling function that may be restored following oyster restoration efforts.

全球贝类礁的受威胁状态已得到充分证实（如Beck等，2011），但此类礁体消失所带来的生态后果仍在很大程度上未被探明。在澳大利亚，贝类礁是受威胁最严重的海洋生境类型之一（Gillies等，2018），其威胁源自历史上的过度捕捞，以及因化肥施用、畜禽养殖与人类废弃物排放导致的近岸水体大范围富营养化。

海洋双壳类是关键的生态系统工程师，在底栖软沉积环境中为其他物种提供栖息场所、庇护载体与食物来源。此外，滤食性双壳类通过滤食与排泄活动，连通了生态系统的底栖与浮游组分。它们通过滤食作用会产生大量粪便（已消化的悬浮颗粒物）与假粪便（被黏液包裹的被拒颗粒），并沉积于底栖环境中。这一过程将浮游系统的能量与养分输送至底栖系统，即底栖-浮游耦合（bentho-pelagic coupling）。大量移除悬浮颗粒物可改善水质与水体透明度，发挥重要的生态系统功能。已有研究证实，双壳类的滤水行为可降低水体浊度、提升光照穿透率，进而改善海草的生长条件（Wall等，2008）。在仍保有健康双壳类种群的生态系统中，其滤水体积可在水体停留时间内达到甚至超过整个河口的容积（zu Ermgassen等，2013）。尽管如今这类高密度牡蛎种群已十分罕见，但该结论凸显了牡蛎礁衰退所丧失的关键生态系统服务。此外，这也揭示了通过牡蛎礁修复可恢复的潜在生态功能。

鉴于人们对这类生态系统衰退的关注度日益提升，针对修复关键生态系统功能的修复行动的研究与实践兴趣也与日俱增。然而，保护与修复决策的制定需依托于相关生态系统服务的可靠量化结果（zu Ermgassen等，2016）。例如，澳大利亚新南威尔士州斯蒂芬斯港正计划修复部分悉尼岩牡蛎（Saccostrea glomerata）的天然牡蛎礁，推动该修复项目的核心动因之一便是恢复已丧失的生态系统服务。

此前已有研究利用添加藻类的过滤水开展水族馆实验，测定了澳大利亚牡蛎的滤水率，但针对使用天然海水的牡蛎滤水率与生物沉积速率的相关研究仍较为匮乏。本研究首次利用天然海水对四种双壳类的滤水率与生物沉积速率进行了评估，以此作为自然生物沉积速率的替代指标。因此，本研究首次揭示了开展牡蛎修复工作后，有望恢复的滤水与养分循环功能。