3.5. Robert's Point

SITE DESCRIPTION

On the afternoon of Wednesday February 23rd, 2011, we conducted conductivity-temperature-depth (CTD) casts at 5 stations at Robert’s Point, off the south-east cost of Robert’s Island in the South Shetland Islands (see figure 34); one of these casts was discarded due to faulty data, leaving casts a total of 4 stations useable for analysis. The weather was overcast with variable winds and brief intermittent periods of sunshine. The water was relatively calm and shallow close to shore, but became choppier as we progressed further away from the coast. Our first station was the closest to shore, in close proximity to a melting glacier. A considerable amount of glacial run-off was observed at this site. The subsequent 3 station were located progressively further from the shore, into deeper water.

Figure 34: Map of Robert’s Point, including the 4 CTD stations

TEMPERATURE

Temperature stratification was observed in the water column at Robert’s Point, with the warmest water occurring closest to the surface, and water temperatures consistently dropping with depth (see figure 35). The warmest recorded water temperatures occurred at our sampling site closest to shore (see figure 36). Unexpectedly, this site was also the closest in proximity to the melting glacier. The fact that warmer water was observed closest to shore is most likely a consequence of the very shallow water depth and the sheltered nature of this area; shallow water masses were easily warmed by the high amount of sunlight received during the summer months. Additionally, the observation that the melting glacier appears to have had little effect on the surrounding water temperature is an indication that the water mass was well mixed.

Figure 35: Temperature gradient at Robert’s Point, moving eastwards away from shore.

Figure 36: Water temperature changes through the depth profile for 4 CTD stations off the coast of Robert’s Point.

SALINITY

The salinity gradient was weak at Robert’s Point, with a maximum salinity change of only ~0.1psu as we descend through the water column. Relatively strong salinity differences were observed at site 1, closest to shore, with the freshest water recorded between all the sites occurring at the top of the water layer at this site, sitting above the more saline water (see figure 37, figure 38) . As we moved away from shore into deeper, rougher and less sheltered waters, we observed progressively smaller differences in salinity throughout the water column. The weak salinity stratification was likely due to the well mixed nature of these waters.

Figure 37: Salinity gradient at Robert’s Point, moving eastwards away from shore.

Figure 38: Salinity through the depth profile for 4 CTD stations off the coast of Robert’s Point.

T/S PLOT

There is a slight negative relationship between potential temperature/salinity (T/S) at Robert’s Point (see figure 39). Density changes are stronger with changing salinity than with changing temperature (i.e. for a given temperature, water density changes significantly as salinity changes, whereas for a given salinity, water density remains relatively constant with changing temperature). This implies that the water column at Robert’s point was stratified primarily by salinity, although both temperature and salinity gradients in this area were extremely weak due to the well mixed nature of the surface ocean layer within which we were measuring. Site 1, closest to shore, exhibited the greatest variation in density, due to the extremely fresh water at the surface. The influx of fresh water at this site is explained by what was likely the presence of a melting glacier on shore.

Figure 39: Relationship between potential temperature and salinity at 4 CTD stations at Robert’s Point.

DENSITY

Density changes were minimal at Robert’s Point, ranging from ~27.07 to ~27.17 (see figure 40, figure 41). This small density range is explained by the weak temperature and salinity gradients present in the water column. Water density is a function of temperature and salinity, with water decreasing in density with decreasing temperatures and increasing salinity; as both salinity and temperature changed very little with depth, density also changed very little with depth. The highest density range was recorded at Site 1, which was to be expected, as this site had relatively strong changes in both temperature and salinity, due to the influx of fresh glacier water, and warming of very shallow waters by the sun.

Figure 40: Potential density gradient at Robert’s Point, moving eastwards away from shore.

Figure 41: Potential density through the depth profile for 4 CTD stations off the coast of Robert’s Point.