Introduction

The observational evidence for persistent thin layers of plankton (e.g. Cowles 2003, Dekshenieks et al. 2001, Holliday et al. 1998) forms the basis for this multi- disciplinary research project. The LOCO project builds upon the findings of the 1998 experiment in East Sound, WA (Figure 1), and will focus on the formation, persistence and horizontal extent of plankotnic thin layers.

Figure 1. Six vertical profiles of phytoplankton fluorescence and sigma_t from a time series of 90 profiles obtained with a free-fall profiling package between 2000 hrs on June 20, 1998 and 0900 hrs on June 21, 1998 at the northern end of East Sound WA. The time of sampling is noted on each panel. A persistent layer of phytoplankton (fluorescence) was observed throughout this 11 hr time period. The arrow indicates the depth of the maximum fluorescence of the layer, and the density (kg m^-3) at that depth is shown just below the arrow. The individual profiles presented in panels A-F are characteristic of the shape, thickness, and variability of this persistent feature during the observation period. This layer occupied a consistent density interval (within 21.86 – 21.88 kg m^-3) throughout the time series. From Cowles 2003.

Our research within the LOCO project will address several key questions about these planktonic layers, including:

What range of conditions results in thin layer formation and persistence?
What are the dominant processes that create and maintain small-scale structure?
Over what time scales do features form, change, or disappear?
Over what horizontal spatial scales do planktonic layers extend?
Are particular coastal and oceanic habitats more or less likely to possess persistent small-scale features?
What is the impact of these features on optical and acoustical signal transduction in the upper ocean?
To what extent, and under what conditions, must we alter our sampling strategies to obtain acceptable, if not perfectly accurate, estimates of the distribution of physical, chemical, and biological properties and rate processes in the upper ocean?

The 2005 and 2006 field experiments will be conducted in Monterey Bay, California, and will involve investigators from several universities and research institutions.

Objectives

These persistent small-scale features result from complex linkages between physical forcing and gradients in the distribution of biological properties over narrow vertical ranges. Our objectives within the LOCO DRI are:

to quantify the linkage between physical processes (mixing and vertical shear) and the steep vertical gradients in biological properties within “thin layers”,
to develop a more detailed understanding of the time scales and horizontal spatial scales over which these physical and biological features persist,
and to evaluate the relative importance of vertical and horizontal processes in establishing planktonic distribution patterns.

These objectives can be met only through the collaborative linkages available within a multi-disciplinary initiative such as LOCO.

Experimental Approach

Our approach to addressing our objectives is to conduct high-resolution vertical profiles of hydrographic, bio-optical, and bio-acoustical properties in conjunction with detailed horizontal mapping of layered properties by other investigators within the LOCO program. Our free-fall profiling system carries multiple instruments (CTD, ac-9s, fluorometers, acoustic Doppler velocimeter (ADV), acoustic Doppler current profiler (ADCP)) and provides centimeter-scale resolution of hydrographic and bio-optical properties. We will also use a newly-developed WideBand High-Frequency Acoustic Profiler (WHAPS) to assess zooplankton aggregations in the vicinity of thin layers of phytoplankton.