Annika Moje, Autun Purser, Laurenz Thomsen, Vikraim Unnithan, Thomas Viergutz Jacobs University Bremen
Objectives
Effects of elevated levels of particulate matter and sedimentation on cold-water coral ecosystems are a matter of great general interest for HERMES. Concern has often been expressed that increased levels of turbidity and sedimentation could have negative effects, e.g. as a result of the re-suspension of sediments by trawling or other human activities close to CWC-habitats, or as a result of eutrophication of overlying surface waters. Aim of the cruise is to study the coral reefs off the Lofoten by using the PADYS (PArticleDYnamics Sensor system (sediment trap, particle sizer, ADCP, CTD, turbidity) and samples (near-bottom water, surface waters) to get detailed information on fluxes of particulate matter through several coral-reefs. Information on particle dynamics will be used to estimate the importance of particle-aggregation, (bio)deposition and (bio) erosion for the coral reefs off the Lofoten. Samples within the reef, from ambient soft bottom communities and from surface waters will be used to trace back the origin of the particles entering the reef-systems. A close collaboration with the MPI group onboard will allow the further investigation of the importance of coral-mucus in the material fluxes.
Fig. 7.1: The PADYS system to study particle dynamics. ADCP, turbidity, CTD, particle sizer, sediment trap
7. HYDRODYNAMICS AND CORAL COMMUNITIES
Work at sea
In total 30 CTD stations, 16 JAGO dives, 3 sensor deployments were used for the analyses. 110 water samples were/are analyzed for Chlorophyll a, turbidity, bioavailabilty and degradation index, particle size, flow direction and velocity, quality of particles (sediment trap), oxygen in near bottom waters. 14 experiments on particle dynamics and mucus production after contact with drill cuttings were carried out.
Preliminary results The Røst Reef
The study area covered about 20 km2 towards the continental slope from water depth of 250 – 750 m. Particle dynamics were studies around the steep, dissected ridges parallel to the shelf break, which were several tens of meters high and showed a characteristic community pattern (described in Hall-Spencer’s report).
The PADYS was deployed twice at the shelf break in water depth of 320 m (18 and 24 hours at two different locations in the northern and southern part of the study site).
Figure 7.3 shows the results of the two ADCP deployments. Flow velocities at 2 m height above seafloor varied during both deployments between 2 and 20 cm/s and directed towards the shelf break and the reef system. Thus, fluxes of particulate matter entered the reef system almost perpendicular and originated from the productive shelf seas.
The progressive vectorplot of the water layers between 5 and 30 m above seafloor revealed that particles entering the reef-system traveled 5 - 10 km over 24 hours, thus passing through the whole reef system. This indicates that the reef community had access to a constant supply of labile organic material from the shelf.
Fig. 7.2: Camera snapshot of aggregates within the reef (copyright to IfM-Geomar)
Surface waters at the Røst Reef had Chl a concentrations of 0.1 – 1.3 µg/l and turbidity ranged from 0.4 - 2.7 NTU (calibrations to follow). Higher concentrations were found in surface and at Sigma 27.5 watermasses. However, the bottom water concentrations
ARK-XXII/1A-C
of chlorophyll a were highly diminished within the reef with maximum concentrations of 0.03 µg/l. The bottom waters in the reef were loaded with large transparent (organic rich) particles which indicate low settling velocities (to be confirmed in the lab). Average particle sizes ranged 2 to > 500 µm. First data interpretation of the size class distribution indicates a shift from fine to coarser particles with increasing proximity to the reef community. All results indicate that the reef community biodeposits most of the labile phytodetritus from the shelf and that the bottom waters within the reef mainly consist of larger particles which aggregated with the finer fraction of the water column. Analyses on the amino acid composition of the material and the determination of the degradation index will give further insight into the particle composition and carbon deposition in this hotspot ecosystem.
Fig. 7.3: Chlorophyl a and turbidity within the water column above the coral-reef. Low concentrations of Chl.a indicate massive biodeposition. The progessive vectorplot-diagram of bottom currents at
the shelf edge indicate cross shelf transport of organic material towards the reef. Red: southern deployment, dark red: northern deployment
One special emphasis of the cruise was to carry out detailed oxygen measurements around and inside the reef. The results show a clear but not yet understood distribution of oxygen in the bottom waters of the reef. As biodeposition seems to be an important process of carbon accumulation at the reef site, oxygen consumption rates in the reef were expected to be very high. Therefore the JAGO submersible was equipped with an oxygen optode to measure variations of oxygen concentrations. Fig 7.4 shows a comparison between two concentration profiles during JAGO dives: one at the Vesteralen site with no coral reef (blue) with little variation in oxygen concentration was found; and one at the Røst Reef (pink) where there is a clear variation of oxygen concentration at different locations within the reef. The data and video-observations from 16 dives will allow us to estimate carbon mineralization rates at different benthic habitats within the reef.
7. HYDRODYNAMICS AND CORAL COMMUNITIES
Fig. 7.4: Comparison between two oxygen concentration profiles during JAGO dives at a non-reef (blue) and reef site (pink)
The Traena Cigar Reefs
The study site covered about 4 km2 at the northern end of a bowl-shaped seabed depression. Within this grid, we carried out 9 CTD/ bottom water sampling stations to determine changes of particle and Chl.a concentrations on down current direction.
PADYS was deployed east of a large cigar reef with live corals to monitor the particle and flow dynamics within a period of 24 h. Flow velocities during the time of deployment varied between 5 and 30 cm/s and were directed towards West and Northwest. The progressive vectorplot of the water layers between 5 and 10 m above seafloor revealed that particles entering the reef-system traveled 5 - 7 km over the period of deployment.
The CTD water sample casts showed decreasing Chl a concentrations of 0.03 – 0.01 µg/l in downstream direction. Turbidity ranged from 1 - 2 NTU and was generally higher than at the Røst site indicating the fluxes of more lithogenic material of fine particle size in the region.
The outer Vesteralen reef site
One JAGO dive at the outer shelf off Vesteralen did not confirm live corals on a hill-like subsea structure and thus could be used a reference station for the oxygen data from the Røst Reef. Fig. 7.4 above shows the oxygen concentrations of the bottom waters at the subsea structures. The oxygen concentrations at this site revealed significantly smaller variation than at the Røst site.
ARK-XXII/1A-C
Drill cutting aggregation in Norwegian waters
For the IRCCM-CORAMM (Coral Risk Assessment Monitoring and Modelling) project 14 surface water samples with phytodetritus were aggregated with drill cuttings under typical surface waters shear conditions to determine the changes of particle size over time. Results indicate a shift of the size fractions towards larger particles and will be used to generate particle transport models for the Norwegian Sea around offshore installations to fine-tune the “zero-emission policy” of the Norwegian hydrocarbon industry.