Welcome to John Marra's Homepage

Lamont-Doherty Earth Observatory of Columbia University
61 RT 9W, Palisades, NY 10964-8000, USA

phone: (845) 365-8891
fax: (845) 365-8150

I am a Doherty Senior Scholar, and part of the Marine Biology group in the Division of Biology and Paleoenvironment at LDEO. My research covers several areas in Biological Oceanography, but particularly concerns the problem of the productivity of the ocean. I want to understand how physical forcing affects the variability of phytoplankton production in ocean ecosystems. As such , I have done research on the effects of monsoons in the Arabian Sea and in Indonesia, the effects of frontal systems, and the springtime restratification in the North Atlantic. I have been involved in the Joint Global Ocean Flux Study in the North Atlantic (1989), the Arabi an Sea (1994-1995) and more recently, in the Southern Ocean (1997-1998).

An important driver of primary production is, of course, solar radiation, and thus, the optical properties of the ocean, and their variability, are a strong interest of mine. Solar radiation entering the sea not only drives photosynthesis, but also heats the ocean, leading to stratification and stability, and therefore a changed environment.

I have two recent manuscripts (submitted) that deal with the issue of productivity in the ocean. The first is an observation of the compensation irradiance (where photosynthesis is ba lanced by respiration). During the Marine Light - Mixed Layers program, we deployed a mooring in the North Atlantic Ocean south of Iceland in early 1989, and observed the inception of the spring bloom. I used the changes in the chlorophyll-a concentration s at particular depths to calculate what the compensation irradiance should be based on the critical depth criterion. My values, at about 0.1-0.3 mol photons per meter squared per day, are at about 1% of surface irradiance.

In the second manuscript, written with Dick Barber (Duke University), we hypothesize a means of calculating phytoplankton and heterotrophic respiration separate from each other. Observa tions of phytoplankton respiration in natural populations are almost non-existent, despite their obvious importance. And, needless to say, it has never been observed with respect to heterotrophic respiration. The method is based on the loss of carbon over night, that is the difference between carbon uptake (based on 14C) from dawn-to-dusk and over 24 hours. We think this is a significant advance.

Other Projects in Progress