rosette in the gyre near Easter Island image

Image courtesy of J. Ras.

Science Plan

General Overview

The South East Pacific (SEP) is characterized by very high nutrient concentrations in the waters adjacent to the Chilean coast, but very low nutrient concentrations (oligotrophic) in the mid- South Pacific Subtropical Gyre (SPSG), near Easter Island. The steep gradient in nutrient concentrations across the region affects the level of marine production, the composition of the microbial community, and the operation of major biogeochemical cycles in ways that are not fully understood. Despite of the remarkable diversity of trophic conditions, strong gradients and even some unique singularities, the SEP is still the most sparsely sampled oceanic region of the global ocean from hydrodynamic, biological and biogeochemical points of view. The SPSG is also the most oligotrophic of all sub-tropical gyres.

Previous expeditions and remote sensing studies have describes the nutrient and chlorophyll field, but there have been few simultaneous measurements of chemical properties with microbial community structure and function.

BiG RAPA proposed cruise track image

Figure 1. Proposed cruise track for the C-MORE 2010 Cruise. Major “process” stations are marked by a yellow star, “survey” stations are marked by a red circle.

Our expedition is designed to investigate the impact of elemental nutrient (nitrogen, phosphorus, iron, silicon, carbon) ratios on marine productivity and microbial community composition. We propose to sample along a line extending from the Chilean coast near Arica to Easter Island (Fig. 1). We will occupy three major “process” stations for up to five days each; a high productivity, near shore station, a mid-cruise station in the nutrient transition zone, and a low productivity, mid-gyre station near Easter Island. In between these stations, we will briefly sample at additional “survey’ stations at lower intensity along the cruise track.

Sampling Plan

At each process station we will deploy and recover an underwater autonomous vehicle to make synoptic measurements of optical, physical and chemical properties of the water column near the sampling station. We will deploy and recover free drifting particle interceptor or sediment traps to measure the vertical flux of particulate matter, free drifting samplers to measure marine productivity and important microbial processes (nitrogen cycling, carbon and oxygen consumption, nutrient uptake). We will also deploy and recover plankton nets and pumping systems to collect particulate matter for chemical and microbial analyses. We will also make extensive use of rosette mounted water bottle samplers to measure water column dissolved and particulate nutrient concentrations, as well as optical, physical, and chemical properties of the water column. Samples for microbial analysis will also be collected.

BiG RAPA proposed cruise track image

Figure 2. Proposed C-MORE cruise track superimposed on a map of November sea surface chlorophyll concentrations, SeaWIFS.

We plan to conduct shipboard incubation experiments to study the effect of changing elemental nutrient ratios on microbial productivity and community structure. Water samples will be amended with nutrients in specific amounts and the changes in chemical and microbial properties will be monitored over time while the samples are maintained in constant temperature incubators located onboard the ship.

We will make a smaller set of measurements at the eight “survey” stations (Fig. 2). These will occur approximately twice each day east of 23°30'W and once a day west of 23°30'W. At survey stations we plan to make two or three short deployments of our rosette-mounted water bottle samples to profile the water column for chemical and microbial properties as mentioned above.

 

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