Seahorse - Bedford Basin Deployment

A map of the Bedford Basin.

A map of the Bedford Basin.

In 1999 Seahorse was deployed in the middle of Bedford Basin, Nova Scotia. Over an 8 week period CTD, turbidity, and chlorophyll concentration profiles were collected in 70 m of water at 4 hour intervals. Vertical resolution was 0.25 m.

Temperature

Temperature readings in the Bedford Basin as taken by Seahorse.

Temperature readings in the Bedford Basin as taken by Seahorse.

As spring progresses, warming of the surface layer is predictable. However, the 2°C drop in the deeper water at the end of March is unexpected. This rapid change followed a period of alongshore winds. Petrie et al. (1987) showed that southwesterly winds cause offshore Ekman transport of surface water which results in upwelling along the coast. This is the likely mechanism for sudden intrusion of cooler water into the basin.

Contour plots from 317 profiles collected in the basin can be viewed to demonstrate the high frequency capabilities of the instrument. Features are revealed that would have been difficult to resolve using traditional sampling methods.

Salinity

Salinity readings in the Bedford Basin as taken by Seahorse.

Salinity readings in the Bedford Basin as taken by Seahorse.

A 0.4 ppt increase in salinity between 10 m and 30 m occurred simultaneously with the sudden temperature change. It is further evidence of intrusion by offshore water.

Increased freshwater following precipitation events is obvious in the surface layer. Drainage of the Sackville River watershed extends the impact of heavy rains over several days.

Turbidity

Turbidity readings in the Bedford Basin as taken by Seahorse.

Turbidity readings in the Bedford Basin as taken by Seahorse.

Turbidity measures water clarity. Elevated turbidities correlate with precipitation events. An extended period of increased turbidity in the top 30 m followed the heavy rain on March 7. Another large turbidity signal followed the rainstorm of March 15.

Clorophyll

Clorophyll levels in the Bedford Basin as taken by Seahorse.

Clorophyll levels in the Bedford Basin as taken by Seahorse.

Large chlorophyll concentration near the surface during the second half of the deployment is indicative of a phytoplankton bloom. It may have resulted from the introduction of nutrients with intrusion of offshore water. Data from the Bedford Basin Monitoring Program show that a large bloom occurred in early to mid-February. The early bloom would have limited productivity by removing nutrients until new ones entered the system.

Increased chlorophyll concentration in deeper water at the end of March coincided with the temperature and salinity change that marked deep water intrusion. This suggests that the source of the deeper chlorophyll signal is a population of phytoplankton transported by the offshore water mass.