Why are we sampling?
Marine ecosystems are not static over time; they may change gradually from year to year or shift abruptly. Changes are in part driven by factors that operate from the “bottom up,” such as environmental changes (e.g., changes in temperature, salinity, or turbidity) and biogeochemical dynamics that affect the availability and recycling of nutrients. Long term monitoring of the area affected by the Exxon Valdez oil spill is important, not only to assess the recovery of resources, but also to understand how the ecosystem is changing over time. This project provides physical and biological measurements that can be used to assess environmental conditions that have bottom-up influences on the marine ecosystems of Prince William Sound.
Where are we sampling?
We deploy an autonomous moored profiler (AMP) in central Prince William Sound and conduct several regular vessel surveys within Prince William Sound (PWS). Vessel surveys visit the mooring site, four historically sampled bays, and the major entrances (Hinchinbrook Entrance and Montague Strait) of PWS. See map at right.
How are we sampling?
The AMP (see the diagram below) measures temperature, salinity, oxygen concentration, chlorophyll-a concentration, turbidity, and nitrate concentration as it ascends from 60 m to the surface each day. In 2016 we added a plankton camera which is an “underwater microscope” that takes pictures of individual plankton as the profiler moves up in the water.
In addition, we conduct vessel surveys six times per year at selected stations throughout Prince William Sound. At each sampling station, we collect water for nutrient and chlorophyll-a analysis, collect plankton, and complete a conductivity-temperature-depth (CTD) cast to collect the same type of data with the same instrumentation as that collected by the instrument on the AMP.
What are we finding?
Analysis of the temperature anomalies (deviations from expected) in Prince William Sound shows a warming trend over the last 40 years at most depths. The temperature trend at the surface is flat, presumably due to enhanced inputs of cold meltwater at the surface along the margin of the Gulf of Alaska.
In late 2013, temperature anomalies shifted to primarily positive, which echoes a pattern of warm anomalies observed across the Gulf of Alaska. That anomaly (colloquially referred to as “The Blob”) is hypothesized to have arisen as a result of a strong atmospheric ridge creating a persistent high pressure of the Gulf of Alaska, which in turn altered storm tracks and resulted in less than average winter cooling. Anomalies within Prince William Sound in 2015 were 4 °C or more above average. The winter of 2015/2016 is also reported to be one of the strongest El Niños on record.
Biological and biogeochemical observations in PWS are much sparser than temperature and salinity, and it is difficult to describe trends without a long time series. The best record currently is satellite chlorophyll (which is confounded by the high degree of cloudiness in the region, as well as by turbidity artifacts), and examination of satellite chlorophyll records from central PWS shows that phytoplankton abundance varies considerably over the course of each year, with many episodic blooms (both during the major spring bloom, and earlier and later in the year). The spring bloom in 2014 (“Blob” year #1) was much earlier and stronger than average, while in 2015 it was comparatively small and late.