1. The distribution of ostebund aggregations (Porifera) in relation to oceanographic processes in the Faroe-Shetland Channel.
Joshua Davison1, Nils Piechaud1, Phil Hosegood1, Kerry L. Howell1
1 Marine Institute, Plymouth University, Drake Circus, Plymouth, UK.
Figure 1: Ostur aggregation in the Faroe-Shetland Channel (JNCC, 2007)
INTRODUCTION
The range of ecological functions and ecosystem services provided by deep-sea sponge aggregations has resulted in these habitats being considered Vulnerable Marine Ecosystems under United Nations General Assembly Resolution 61/105. Understanding the distribution of these habitats is critical to future spatial management efforts, and the key to predicting their distribution lies in understanding the role of environmental drivers. Accumulations of large suspension feeders are hypothesised to aggregate near the shelf break in regions of internal wave breaking. The causal link is thought to be an increase in the supply of food related to the incidence of internal waves which results in resuspension of particulate organic matter on which the sponges feed. We tested the relationship between sponge density and oceanographic variability (as a measure of internal wave presence) for a known sponge aggregation in the Faroe-Shetland Channel.
HYPOTHESES:
H1 – The abundance of Porifera is associated with the presence of internal waves.
H2 – The abundance of Porifera is associated with the resuspension of particulate.
METHODOLOGY:
Porifera abundance derived from 225 images attributed to 28 transects ranging from m depth. Images were analysed and all taxa identified to morphotypes and quantified (Fig. 2).
Temperature and salinity data obtained from 36 CTD casts (BODC data) spanning 28 years ( ) (Fig. 2).
Particulate flux data were obtained from two different PROCS cruises (PROCS-1/3) in spring/autumn 1999 (Hosegood, 2005). Measured using sediment traps and optical backscatter sensors (OBS)
Least-Squares linear regression models between the abundance of Porifera ~ the variation in temperature range; an indicator of IWS, and OBS data.
Figure 2: Site map detailing the locations of CTD casts (Black points), Image transects (White points) grouped by sample site (FSC_N/S) and the PROCS-1/3 mooring site (Black thick line (Adapted from Hosegood, 2005)).
RESULTS:
Highest variation in temperature at ~430m (range of 9.17°C), steadily decreases to 0°C at ~1000m, indicating the presence of internal waves at ~430m (Fig. 3).
Highest abundance of Porifera at 504m (n=361) highest abundances across sites were shown to be between ~ m (Fig. 3), after the peak at 500m there is a sharp decline in sponge abundance down to ~1000m.
Variation in temperature was strongly positively correlated with Porifera abundance (F1,23 =20.55, ŷ= , p<0.001, R2=0.44).
PROCS-1 OBS values are weakly negatively correlated with Porifera abundance (F1,23=82, ŷ= , p<0.05, R2=0.14) (Fig. 4).
PROCS-3 OBS values are strongly positively correlated with the Porifera abundance (F1,23=12.82, ŷ= , p<0.01, R2=0.33).
The highest PROCS-1 OBS values were at the shallowest mooring (548m) followed by a secondary much smaller peak at 898m, overall there is a negative trend in OBS from 548m to 998m (Fig. 3).
CONCLUSIONS & DISCUSSION:
The model showed a statistically significant relationship between sponge density and temperature variation, with the highest sponge densities occurring at depths of greatest temperature variability.
Greatest particulate flux and current velocity occurred deeper than the depth of known sponge aggregations suggesting these areas are sub optimal for sponge aggregation presence. Our data broadly support current theory on drivers of deep sea sponge aggregation distribution.
Figure 3 - Abundance of Porifera for each of the image transects and Environmental variables plotted against depth
Figure 4: Characteristic lamellate and lobose sponges of the observed ostebund aggregations in the Faroe-Shetland Channel (JNCC, 2007)
ACKNOWLEDGEMENTS:
Financial support by the from UK Department for Trade and Industry and Plymouth University is gratefully acknowledged. CTD data were obtained with from the British Oceanographic Data Centre under NERC open data licences (Open Government License v1.0). Particulate data were obtained from the PROCS series of cruises undertaken by the Royal Netherlands Institute of Sea Research (NIOZ). Deep sea images were taken from the MV Franklin Cruise 0206.