4. Site Inter-comparisons

When we compared the conductivity-temperature-depth (CTD) data points from the localities we visited in 2011, we immediately observed some differences between sites. At Neko Harbour, colder water occurred at shallower depths than warmer water in the water column, implying that stratification at this site is primarily a function of salinity, rather than temperature. A similar trend was observed in Paradise Bay, with colder water rising above warmer water at some stations due to a stronger salinity gradient. There was a high number of 'bergy bits' and ice present at Paradise Bay at the time of measurement, which had calved off from the surrounding glaciers and likely contributed to the presence of these cold, fresh, pockets of water. A pocket of slightly warmer water was also observed near the bottom of the depth profile for Brown Bluff, although the trend was less dramatic than at either Neko Harbour or Paradise Bay and may have been a result of the melting of the large surrounding icebergs that were next to the sampling area. In this respect, the other three shallow-cast sites - Robert’s Point, the Errera Channel, and Pendulum Cove - differed from the three aforementioned sites, as their data provided consistent results with regards to temperature and salinity stratification (i.e. temperatures decreased with depth, and salinity increased with depth). This could be due to the warming effects of the sun on the photic zone in these relatively stable water columns, and the absence of icebergs away from shore. Our two deep-cast stations taken in the Antarctic Sound and Gerlache Strait also showed stable temperature stratification, with temperatures decreasing with depth.

In comparison to the other locations, Robert’s Point appears to be weakly stratified with respect to both temperature and salinity, with fairly uniform values throughout the water column. It is likely that the shallow water depth and sheltered environment contributes to this weak stratification. This could be simply due to the fact that our cast did not descend past the mixed layer. Similar to our data taken at Robert’s Point, the second CTD cast conducted at Pendulum Cove also showed weak temperature and salinity stratification. Again, the results from this station may have been influenced by our CTD not fully penetrating the mixed layer. Alternatively, this location may have been affected by Deception Island’s geothermal activity or influenced by water circulation within the harbour. The other shallow-cast sites, which consisted of Brown Bluff, the Errera Channel, Neko Harbour, Paradise Bay, and the first station at Pendulum Cove, had increased levels of temperature stratification. It should be noted that the Paradise Bay data provided atypical temperature stratification results, which we believe is due to the influence of the 'bergy bits' and glaciers, as well as the high calving activity that took place very close to our stations. The data from our deep-cast locations also shows that the Antarctic Sound had lower temperature stratification than the Gerlache Strait.

A comparison of potential temperature-salinity plots implies that Neko Harbour is anomalous to the other sites we visited with respect to temperature and salinity. At Neko Harbour we observed a positive relationship between potential temperature and salinity at all 3 locations (i.e. temperature increases as salinity increases). While the stations showed pockets of colder water above warmer water at Paradise Bay (and to a lesser extent at Brown Bluff) these trends were not ubiquitous at all sites, and not as strong as at Neko Harbour, as can be observed by comparing the potential temperature and salinity relationships at each site. The temperature-salinity plots of the other shallow-cast sites, located at the Errera Channel, Robert’s Point, and Pendulum Cove, illustrate the occurrences of more typical stratification (where temperature decreases and salinity increases as depth increases). Our results from the Antarctic Sound and the Gerlache Strait show that the waters of these two deep-cast locations share the same general indirect relationship between potential temperature and salinity as the Errera Channel, Robert’s Point, and Pendulum Cove.

At Robert’s Point, we observed the warmest water closest to the melting glacier, which was likely partially due to the fact that the area was very shallow and sheltered. This difference in water temperatures is also likely a function of the ice cover of the respective sites. At Neko Harbour and Paradise Bay, we observed a considerable amount of ice, whereas Robert’s Point and Pendulum Cove were largely ice-free. Brown Bluff and the Errera Channel did not have ice cover on the water like Neko Harbour and Paradise Bay, but had sites which were in close proximity to glaciers and icebergs. Since heat is transferred from water to ice, areas with large ice cover, such as Neko Harbour and Paradise Bay, would thus be expected to have colder water temperatures, as we observed.

Through analysis of our deep-cast locations, we observed a much larger temperature range in the Gerlache Strait than in the Antarctic Sound, which was mostly due to the fact that the latter location is in a more open area of the sea and is thus more influenced by the Southern Ocean. Its position means that it is more difficult for the waters to warm, and because of the slightly higher salinity and the stronger current, the waters are able to remain below 0°C without freezing. Also, our CTD cast in the Antarctic Sound was taken from within a few hundred meters of a large iceberg, which likely contributed to the very cold waters found throughout the water column. Meanwhile, the Gerlache Strait location was more isolated from winds and from the Southern Ocean than the Antarctic Sound, and thus may have exhibited a greater response to warming from the sun or other factors.

We also observed a comparatively stronger current in the Errera Channel than in the other shallow-cast locations. We noticed the existence of the current through observations that a density gradient was present, and we believe that this current is most likely due to tides or wind. We did not observe any significant indications of currents at Brown Bluff, Neko Harbour, Paradise Bay, or Pendulum Cove; although, it should be noted that at Robert’s Point we noticed an angle formed by the CDT rope’s actual position and the ideal 90° position (i.e. the CDT following a path straight down towards the sea floor), which means that there was a slight current present. We believe that this was likely the influence of tides around Robert’s Point or the influence of the Southern Ocean current. Both of our deep-casts, taken in the Antarctic Sound and the Gerlache Strait, were recorded in the presence of high currents. In fact, the currents in both of these locations proved to be too strong to record Secchi depth readings without damaging the Secchi Disk.

Table 4: Symbols representing each station visited on the 2011 Antarctic expedition

Figure 58: Potential temperature vs. salinity for all stations, 2009 and 2011. Isopycnals are shown by grey lines. Weddell Sea sites plot in the cold and salty part of the diagram, South Shetland sites plot in the warm, intermediate salinity part of the diagram, and the western Antarctic peninsula sites plot in the fresh, intermediate temperature part of the diagram. Locations are shown in corresponding symbols on the map.

WATER QUALITY COMPARISON

Our Secchi Disk measurements provide specific data about each of the three sites where we took them, and highlight differences between the locations as well. We were able to determine that the overall productivity at Brown Bluff was fairly low at all five of our stations as deep Secchi depth values were observed. In chronological order, our sites measured 10 meters, 12 meters, 13 meters, 15 meters and 17.5 meters. These values explain the depth to which visible light penetrates the water. The weather conditions at Brown Bluff may have slightly impacted our results, as it was a very sunny day with with clear skies and thus more sunlight was entering the water. When we took measurements in the Errera Channel on February 21, 2001, the weather conditions were very similar, yet despite high amounts of sunlight and relatively clear skies, our data was quite different from that retrieved at Brown Bluff. In the Errera Channel, our Secchi depth readings from the seven stations were 6 meters, 6.5 meters, 7 meters, 6 meters, 6.5 meters, 7 meters, and 6 meters respectively. Although our stations nearly spanned the channel from the first to the last, which covered a sizeable distance, the depth that light penetrated did not vary more than a meter between sites. This indicates that the Errera Channel was experiencing high productivity in its photic layer, which inhibited visible light from penetrating deeper. Thus, under very similar conditions in both locations, we observed the lowest and highest productivity levels with our Secchi Disk readings at Brown Bluff and the Errera Channel respectively. The third location where we recorded Secchi depths was at Neko Harbour on the morning of February 22, 2011. Unlike the other two sites, the weather here was overcast with thick clouds, which allowed for much lower levels of light to reach the water. Still, the Secchi Disk readings provided us with useful information, namely that the waters had fairly low productivity. Despite the lack of direct sunlight, our stations at Neko Harbour measured Secchi depths of 10 meters, 9 meters, and 8 meters (in chronological order). We conclude that Neko Harbour has fairly low levels of productivity, falling in between the results of Brown Bluff and the Errera Channel. One final notable inter-comparison that we observed was that while there was practically no variation between the Secchi depths of our different stations in the Errera Channel, location played a larger role at the other two sites. This was extremely apparent at Brown Bluff. As we moved further from the beach, which was inhabited by a large number of Gentoo penguins and Fur seals, and closer to several large icebergs, the Secchi depths increased dramatically. Yet, at Neko Harbour we observed the opposite trend in our data; although our three stations here were not as far apart as the station spacing was at Brown Bluff and the Errera Channel, we noticed that light penetration decreased as we moved farther from the shore, which held a sizable Gentoo penguin rookery.