SEAPLEX

Our second post today is from Pete Davison.

He writes:

The past two weeks of the SEAPLEX cruise have been spent far offshore in oligotrophic (nutrient-poor) water. In the California Current closer to shore, the nutrients required for plant growth are present in the surface water. This cold, nutrient-rich water has been recently upwelled from the deep by the action of winds blowing south along the coast. The growth of phytoplankton near the surface depletes the nutrients as the water warms up. Primary productivity is a measure of the rate of growth of phytoplankton. It is typically measured in grams of carbon per square meter per day. The carbon comes from photosynthesis as the plants change CO2 to O2 with energy from the sun. The winds eventually push the warmer, nutrient-depleted surface water offshore to the west. The position of the SEAPLEX trawls has been marked on a satellite map of annual primary productivity from our study area in the image below.

Phytoplankton are consumed by zooplankton, which are in turn eaten by larger predators. Changes in the pattern of food availability delineate pelagic habitat changes, and are reflected in the communities of animals that live there. The amount of available food also determines the quantity of animals (biomass) that can be supported in the habitat.

We have been collecting very small volumes of zooplankton and fish at Stations 2, 3, and 4 that correspond to the low productivity of the habitat we are sampling in comparison to Station 1 in the California Current. We have also been catching different species of animals that reflect the new habitat that we are sampling. This is clear in the photo below.

We separate the fish from the zooplankton at sea before preservation because two groups of animals are often studied by different scientists.  From the map, annual primary productivity at Station 1 is ~1 gram of carbon per square meter in comparison to ~0.3 grams of carbon per square meter at Station 4. We would expect a ratio of ~3.3 in the catch volume (assuming that the standing stock of animals is equal to annual growth). It can be seen that Station 1 had roughly four times the zooplankton volume and twice the fish volume as were found at Station 4. This is pretty close to our expectations.

Map of primarpy productivity in the Eastern North Pacific Ocean.   The SEAPLEX cruise track is overlayed on the map.  Station 1 is in the California Current region, an area of high primary productivty.  Stations 2, 3, and 4 are in the North Pacific Gyre, an area of low primary productivity.

Samples collected on the SEAPLEX cruise.  The samples were divided into zooplankton (left) and fish (right).  The jars in the back are from one trawl at station 1 in the California Current.  The jars in the front are from a similar trawl (same duration, same speed, same net, etc.) from Station 4 in the North Pacific Gyre.