Ashastina, Kseniia (2013) Water mass dynamics and biogeochemistry of the cold-water coral reef, Stjernsund, northern Norway. (Master thesis), Saint-Petersburg State University ; University of Hamburg, Saint Petersburg, Russia ; Hamburg, 86 pp.

Preview

Text 2013_Ashastina-Kseniia_MSc_Thesis.pdf - Accepted Version Available under License Creative Commons: Attribution-Noncommercial 4.0. Download (4MB) | Preview

Abstract

Cold-water corals are an object of world-wide research. Active exploration of the reef-building coral species has begun in the XX century with the development of the digital sea bottom mapping techniques and determines that cold-water corals can thrive in a wide range of depth below the photic zone from 43 m down to 3 380 m, but crucial ecological parameters governing the distribution are still not fully understood. In the focus of this study lies the northernmost living cold-water coral reef, known so far, which is located in the Stjernsund (70: 15′ N 22: 20′ E), Northern Norway. The main coral reef is formed by the scleractinia Lophelia pertusa. An underwater 4D Modular Laboratory, compiled of 5 landers and 2 moorings, was especially designed at the GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany, to investigate the living coral communities of the Stjernsund. The elongated structure of the sound extends from SE to NW, connecting the North Atlantic with the Altafjord, is 30 km long and 3.5 km wide with the sill structure across it. From 1.06.2012 to 15.09.2012 the underwater laboratory was installed (434 and 438 POSEIDON cruises) on the sill recording oceanographically and biogeochemical data. Underwater observatory modules were installed on both slopes of the sill (one mooring and one lander on each slope) and on top of the sill (3 landers), so that the entire sill area was covered with measurements and water column, depending on the module settings, provides ADCP records. The Modular laboratory recorded the following physical and biochemical parameters: velocities and directions of the water flows, temperature, salinity, pH, turbidity, fluorescence, oxygen concentration and saturation. In this work data sets from 7 modules are analyzed in order to determine the ecological factors governing the uneven distribution of the living corals on the slopes of the sill and in order to reconstruct the water mass dynamics in the sound.
The raw data was processed with the software Matlab. Results showed that near-bottom salinities, temperature and current velocities indicate a semi-diurnal tidal forcing (pronounced M2 constituent), which cause vertical water mass movements of up to 100 m, that influences large parts of the living reef. Reconstruction of the residual current flow in the sound has given an idea of the real water flow in the northern setting without tidal influence. Detailed examination of the water mass motions both diurnal and entire observation period has confirmed suggested theory The raw data was processed with the software Matlab. Results showed that near-bottom salinities, temperature and current velocities indicate a semi-diurnal tidal forcing (pronounced M2 constituent), which cause vertical water mass movements of up to 100 m, that influences large parts of the living reef. Reconstruction of the residual current flow in the sound has given an idea of the real water flow in the northern setting without tidal influence. Detailed examination of the water mass motions both diurnal and entire observation period has confirmed suggested theory stating that a turbulent cell occurs on the south-eastern slope of the sill at high tide conditions, when the Atlantic Water crosses over the sill.
To investigate fluctuations of the ecological parameters and determine its influence on the corals, near-bottom water layers were analyzed with a daily resolution and over the entire observation period. Diurnal observations revealed that changes in temperature never exceeded 0.2 :C, salinity during the day changed not greater than 0.3 psu. Oxygen loggers revealed values in the range 10 μM and 4% for concentration and saturation respectively. Appearance of living cold-water corals lies within density envelope 27.25-27.50 kg m³, which marks the boundary between Norwegian Coastal Water and Atlantic Water.
Lophelia pertusa as a globally distributed species can live in a wide range of physical and geochemical parameters. Observed data sets of the entire period present the agreeable with other explored living coral locations ranges of values of current velocities (15-30 cm/s), temperature (6.0-6.8:C) and salinity (34.1-34.8 ‰), pH (8.22-7.39), turbidity (0.1-0.9 NTU), fluorescence (0.04-0.16 mg/m³) and oxygen concentration (300-339 μM). Only the pH curve showed an unusual pattern unobserved at other locations. The pH value shows a linear decreasing trend during the summer period, reaching 7.4 pH. In this study several possible reasons for this drop are explored as the pH magnitude 7.6 is crucial for Lophelia reefs, this pH triggers dissolution process.
Water flow in the Stjernsund, Northern Norway, depicts a complex dynamic system with pronounced differences not only vertically, but also important horizontal changes on top of the sill.
Most likely that uneven distribution of living corals is influenced in greater value by parameters that have not been covered in this work. In this context the water dynamics, especially water motions over the sill, bringing Atlantic Water in the SE part of the sound, seem to play an essential role.

Actions (login required)

View Item