THE PENETRATION OF THE BLUE AND RED COMPONENTS OF DAYLIGHT INTO ATLANTIC COASTAL WATERS AND ITS RELATION TO PHYTOPLANKTON METABOLISM

¹ From the Woods Hole Oceanographic Institution, and the Laboratory of General Physiology, Harvard University

1. Photoelectric measurements of blue and red submarine irradiation were made at two stations in the Gulf of Maine and at four localities in the Woods Hole region. The same apparatus was used as before (Clarke, 1933) with the addition of two cæsium on cæsium oxide photocells cells mounted under Schott-Jena RG-1 filters and CuCl₂ water-cells which limited the radiation received to 6000Å to 7000Å.

2. The bodies of water considered may be arranged in order of decreasing transparency to both blue and red light as follows: (a) the deep basin of the Gulf of Maine, (b) Georges Bank, (c) Off Gay Head, (d) Vineyard Sound, (e) Buzzards Bay and Woods Hole Harbor. The ratios of the transmissive exponents for red light to those for blue light in each locality vary from 2.9 to 1.2, the higher values being characteristic of clearer water.

3. These measurements are discussed in relation to the local hydrographic conditions, and are compared with observations made in the Plymouth region and in the San Juan Archipelago.

4. Measurements made by Dr. H. H. Gran on the metabolism of diatoms in bottles suspended at different depths in Woods Hole Harbor indicated that the "compensation point" was located at a depth of about 7 meters. Similar measurements carried out by Dr. A. C. Redfield gave a depth of 24-30 meters for the "compensation point" in the Gulf of Maine.

5. Observations reported by Mr. T. Braarud (1934) on the vertical distribution of the phytoplankton at the same stations in the Gulf of Maine are discussed in relation to the penetration of light.