Drivers of epibenthic megafaunal composition in the sponge grounds of the Sackville Spur, northwest Atlantic

Deep-water sponges are considered ecosystem engineers, and the presence of large aggregations of these organisms, commonly referred to as sponge grounds, is associated with enhanced biodiversity and abundance of epibenthic fauna compared to non-sponge habitat. However, the degree and magnitude to which the presence of these sponge grounds elicits large changes in composition of the associated megafaunal community remains unknown. Here we identify the external drivers of epibenthic megafaunal community composition and explore the patterns and magnitude of compositional change in the megafaunal community within the sponge grounds of the Sackville Spur, northwest Atlantic. Epibenthic megafauna were quantified from five image transects collected on the Sackville Spur in 2009 between 1080 and 1723 m depth. Using Gradient Forest Modelling we found that the abundance of structure-forming sponges was the most important variable for predicting compositional patterns in the Sackville Spur megafaunal community, followed by depth, range in bottom current speed, in situ salinity,and longitude. Along the gradient in structure-forming sponge abundance, the largest turnover in megafaunal community composition occurred when the sponges reached 15 individuals m−2. Examination of the regional hydrographic conditions suggests that the dense sponge grounds of the Sackville Spur are associated with a warm, salty water mass that occurs between ~ 1300 and 1800 m.

深海海绵被视为生态系统工程师，这类生物的大型聚集体通常被称为海绵床（sponge grounds）。相较于无海绵栖息的生境，海绵床的存在可提升当地生物多样性以及底上大型宏生物（epibenthic megafauna）的种群丰度。然而，此类海绵床的存在会在多大程度与幅度上引发与之关联的大型宏生物群落组成发生显著改变，目前仍未明确。本研究明确了调控底上大型宏生物群落组成的外部驱动因子，并探究了西北大西洋萨克维尔海丘（Sackville Spur）海绵床内大型宏生物群落的组成变化模式与幅度。研究基于2009年在萨克维尔海丘1080至1723米水深范围内采集的5条图像样带，对底上大型宏生物进行了定量分析。本研究采用梯度森林模型（Gradient Forest Modelling）开展分析，结果显示造构海绵丰度是预测萨克维尔海丘大型宏生物群落组成模式的最重要变量，其次依次为水深、底流速度变幅、原位盐度与经度。随着造构海绵丰度的梯度变化，当海绵密度达到15个·m⁻²时，大型宏生物群落组成发生了最为显著的更替。对区域水文条件的分析表明，萨克维尔海丘的高密度海绵床与1300至1800米水深范围内的暖盐水团存在显著关联。