Abstract
The Scotian Shelf harbors unique aggregations of the glass sponge Vazella pourtalesii that provides an important habitat for benthic and pelagic fauna. Recent studies have shown that these sponge grounds have persisted in the face of strong inter-annual and multi-decadal variability in temperature and salinity. However, little is known of these environmental characteristics on hourly-seasonal time scales. This study presents the first hydrodynamic observations and associated (food) particle supply mechanisms for the Vazella sponge grounds, highlighting the influence of natural variability in environmental conditions on sponge growth and resilience. Near-bottom environmental conditions were characterized by high temporal resolution data collected with a benthic lander, deployed during a period of 10 months in the Sambro Bank Sponge Conservation Area. The lander was equipped with temperature and oxygen sensors, a current meter, a sediment trap and a video camera. In addition, water column profiles of temperature and salinity were collected in an array across the sponge grounds from high to lower sponge presence probability. Over the course of the lander deployment, temperature fluctuated between 8.8–12°C with an average of 10.6 ± 0.4°C. Dissolved oxygen concentration was on average 6.3 mg l–1, and near-bottom current speed was on average 0.12 m s–1, with peaks up to 0.47 m s–1. Semi-diurnal tidal currents promoted constant resuspension of particulate matter in the benthic boundary layer. Surface storm events episodically caused extremely turbid conditions on the seafloor that persisted for several days, with particles being resuspended to more than 13 m above the seabed. The carbon flux in the near-bottom sediment trap peaked during storm events and also after a spring bloom in April, when fresh phytodetritus was observed in the bottom boundary layer. While resuspension events can represent a major stressor for sponges, limiting their filtration capability and remobilizing them, episodes of strong currents and lateral particle transport likely play an important role in food supply and the replenishment of nutrients and oxygen. Our results contextualize human-induced threats such as bottom fishing and climate change by providing more knowledge of the natural environmental conditions under which sponge grounds persist.
Highlights
Sponges are one of the most common megafaunal organisms in the deep sea (Tabachnick et al, 1994)
The dissolved oxygen (DO) concentration was negatively correlated with density (r2 = -0.52, p < 0.001) and was on average highest in spring from March to June (6.7–7.1 mg l−1), while lowest values were found from September to February (6.0 – 6.2 mg l−1, Figure 6A)
Resiliency against environmental and anthropogenic stressors requires that the surrounding environmental conditions, such as temperature, DO concentration and food supply, be within a species’ optimal physiological tolerance limits (Odum, 1971; Sokolova et al, 2012)
Summary
Sponges are one of the most common megafaunal organisms in the deep sea (Tabachnick et al, 1994) They can have a scattered distribution with low abundances, but under certain environmental conditions may form dense monospecific or mixed sponge aggregations, and even reef-like ecosystems (Maldonado et al, 2017). Sponges play a key ecological role in benthic-pelagic coupling by filtering substantial volumes of water (up to 24,000 L per kg−1 of sponge day−1 in temperate and tropical shallowwater sponge grounds; Leys et al, 2007; Maldonado et al, 2012; Pham et al, 2019) They can release particulate detritus to the associated benthic environment (Reiswig, 1971), a pathway referred to as the sponge loop (Rix et al, 2016). They transfer energy and nutrients from the pelagic to the benthic environment, and play an important role in carbon, nitrogen, phosphate and silicon cycling (Maldonado et al, 2012; Kutti et al, 2013; Cathalot et al, 2015; Maldonado et al, 2020)
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