Benthic jellyfish dominate water mixing in mangrove ecosystems

Abstract

Water mixing affects aquatic ecosystem processes by regulating the interactions between water masses such as delivering nutrient-rich water to depleted areas. Physical processes, such as winds or tides, are primarily responsible for mixing effects in shallow coastal systems, but the sheltered flats adjacent to mangrove habitats experience very low turbulence and vertical mixing. The significance of animal biogenic mixing in open ocean habitats is unclear, but in this study we explore the role of the upside-down jellyfish Cassiopea sp., with respect to biogenic contribution to water column mixing in shallow, sheltered habitats. The mixing contribution was determined by means of high definition Particle Image Velocimetry (PIV) in both the laboratory and in the natural environment. Cassiopea sp. was found to continuously pump water from the benthos upward in a vertical jet with flow velocities in the range of 1 to 2 cm s -1 . The volumetric flow rate was found to be 212 l h -1 for average sized animals (8.6 cm bell diameter), leading to a water column turnover rate of about 15 minutes for a median population density (29 animals m -2 ). In addition, Cassiopea sp. were responsible for release of porewater into the water column at an average rate of 2.64 ml h -1 . This release of benthic porewater, which is often nutrient-rich, in conjunction with the increase in water column mixing, suggest a role for Cassiopea sp. as an ecosystem engineer in mangrove habitats.Cassiopea | Upside-down Jellyfish | Water Mixing | Particle Image Velocimetry | Nutrient Flux

Aggregation and Defence

“…While this slow movement may aid in finding a more favourable position within a mangrove habitat Cassiopea sp. require nutrients and must consume benthic prey and/or release interstitial, nutrient-rich porewater (Durieux et al, 2021). Since prey and nutrients could be easily depleted if permanently fixed in one location, one potential use of slow movement across the benthos would be as a means to bolster nutrient or food intake by moving the animal onto fresh habitat that has not been depleted of these resources.…”

“…Cassiopea sp. are known to occur in clumped distributions (Durieux et al, 2021; Niggl & Wild, 2010), with small individuals aggregating more densely (Tsingalia, 2014). Although this aggregation behaviour is well documented, the mechanism has not been investigated.…”

“…This physical contact has been shown to reduce the volumetric flux of the feeding current in Cassiopea sp. (Durieux et al, 2021) due to interference between the horizontal incurrent flows of multiple animals, which was aggravated as additional neighbours were added. This is due to the horizontal incurrent flow, which flows along the benthos radially towards the jellyfish (Durieux et al, 2021).…”

“…were found to have an average density of 1.6 animals m −2 , while some areas had up to 31 animals m −2 (Niggl & Wild, 2010). Higher densities have been observed in the Florida Keys, up to 97 animals m −2 where the median value was 29 animals m −2 (Durieux et al, 2021). Nearby regions are often devoid of Cassiopea sp.…”

“…In this position, they pulse their bells in a motion similar to the swimming mechanism of other jellyfish, but rather than displacing their bodies, they create a feeding current which draws prey from the surrounding water into their filter feeding arms. This water is then ejected upwards into a vertical jet with velocities up to 1-2 cm s − 1 (Durieux et al, 2021). Cassiopea sp.…”