Where Light and Nutrients Collide: The Global Distribution and Activity of Subsurface Chlorophyll Maximum Layers

Abstract

A satellite view of the world’s oceans presents a mosaic of high chlorophyll (Chla) regions adjacent vast Chla deserts. However, such a view is limited because it reflects conditions of near surface waters only, and misses the vast and sometimes permanent regions of elevated Chla that can exist in subsurface waters. Subsurface chlorophyll maximum layers (SCMLs) are widespread features of the global ocean and are composed of phytoplankton communities that are chromatically and nutritionally adapted to these environments. In this chapter, we first outline the drivers that structure the formation and persistence of SCMLs in marine systems. We develop a simple model that predicts the global distribution and seasonal persistence of SCMLs and find that during any given season, between 59 and 73 % of the ocean may support an SCML. Using a well established global net primary production model, we further predict that approximately 47 % of ocean primary production occurs within SCMLs, a surprisingly large fraction, given the degree of light limitation at these depths. For context, we synthesize key works that have investigated primary production, phytoplankton biomass, and/or nutrient turnover within SCMLs across a range of ocean biomes. These recent studies support previous hypotheses that SCMLs are important sites for new production, and indicate that this new production largely occurs during times when SCMLs are moving deeper into nutriclines or when they are supplied with nutrients through other mechanisms (e.g., tides). In a final section, we draw upon our formative studies in limnology to make linkages between marine and lacustrine systems in terms of the structure and function of SCMLs. Because of large gradients in size, optical properties, and nutritional status across lakes, these systems may present ideal environments to test hypotheses related to the regulation and consequences of SCML productivity.

Greg M. Silsbe and Sairah Y. Malkin

We met the day Sairah was interviewing for an M.Sc. position with Bill Taylor at the University of Waterloo. In the hopes of impressing her, I (Greg) informed her I had just returned from a CIDA-funded internship program on the tropical shores of Lake Victoria, East Africa, where I was soon to be returning as an M.Sc. student. And then, I proceeded to show her data tables of CTD profiles. Nevertheless, we would be dating a year later, and we remained at U. Waterloo, to pursue Ph.D.s in limnology with Bob Hecky and Stephanie Guildford, supervisors whose wisdom and humanity has informed much of our careers. Bob had once suggested that if we wanted to better understand lakes, we should take a look at coastal oceans. We took this to heart, and so from U. Waterloo, we went on to pursue postdocs at The Netherlands Institute of Sea Research (NIOZ; with Jacco Kromkamp and Filip Meysman), living variously in The Netherlands, and then across the Schelde in Antwerp, Belgium (where the roads were worse, but the food was better). We were becoming increasingly specialized in the benthos (Sairah) and pelagia (Greg). Logically, our first manuscript together would examine benthic-pelagic coupling (Malkin et al. 2012). After brief stops at the University of Georgia (Sairah; with Mandy Joye), and the University of Oregon (Greg; with Toby Westberry), we are now about to embark on faculty positions with the University of Maryland Center for Environmental Sciences (UMCES), Horn Point Laboratory. We are grateful to institutes that recognize the synergistic benefits of dual spousal hires, and we are looking forward to continuing our research careers together at UMCES.