In an attempt to establish correlations between variations in the abundance of certain marine organisms and fluctuations in hydrographic variables, a program was initiated late in 1950 that involved periodic sampling of water and plankton from the surface to depths of 1000 meters. The physico-chemical variables of temperature, salinity, oxygen, silicate and phosphate were determined at intervals from the surface to a depth of 1000 meters for the first 2 years of the program, and to 900 meters subsequently when the loss of some of the hydrographic wire necessitated limiting the depth of the casts. This report presents the hydrographic data. (Author).

1. The marine climate along the coast of central California is characterized by three more or less well-defined seasons. These are the result of the alternating water flow induced largely by the prevailing winds. During the period of northwesterlies the surface water is driven to the south and offshore, and upwelling deep water replaces it. The subsequent period of calms is marked by a superficial inflow of oceanic surface water. In winter the southerly winds induce the Davidson Current which flows northward and impinges on the coast. The water during each of these seasons differs from that characteristic of other seasons by recognizable features of temperature and salinity. 2. The Davidson Current period (November to February) is marked by (1) an abrupt decrease followed by a continuing decline in surface temperatures, (2) homogeneous surface temperatures over Monterey Bay, (3) a thermal gradient of usually less than 1°C. between the surface and a depth of 50 m. throughout the bay, (4) temperatures at 50 m. at the high point of the year, and (5) surface salinities at the low point of the year. 3. The upwelling period (February to September) is marked (1) surface temperatures reaching the low point of the year, then rising to approach the annual high, (2) surface temperatures showing a spread of 1 to 3°C. or more in different parts of the bay, (3) a thermal gradient of 3°C. or more between the surface and 50 m., (4) temperatures at 50 m. reaching the low point of the year, and (5) surface salinities rising to the high point of the year and beginning to decline. 4. The oceanic period (September and October) is marked by (1) surface temperatures at the high point of the year, and exhibiting a range of 2°C. or less in different part of the bay, (2) persistence of a strong thermal gradient between the surface and 50 m., often with a clear thermocline, (3) temperature values at all levels from surface to 50 m. 2-3°C. higher than during the peak of upwelling, and (5) salinities tending to rise slightly. 5. While the general pattern of seasons is repeated annually, marked differences do occur from year to year. These differences are discussed for each individual year during the period 1954-1960. 6. In addition to a clear annual temperature cycle a much longer temperature trend is recognizable, which reached its warmest point in 1958. 7. The phytoplankton undergoes an annual cycle of increase and decrease in standing-crop volume, with peak volumes tending to occur in June. The relationship between plankton volumes and such variables as nutrients, and direction and intensity of water flow are discussed. In general, high plankton volumes occur during periods of upwelling. It appears that steadiness of upwelling is a major factor influencing the magnitude of coastal plankton blooms: if upwelling is intermittant, coastal plankton volumes are high; if it is steady, volumes are lower, probably as a result of horizontal divergence of coastal surface waters rather than low production. 8. The beginning of each marine season is marked by a rather abrupt increase in the taxonomic complexity of the phytoplankton, with many genera and species represented simultaneously in the catches. As the season develops, certain forms achieve dominance over the others, and the plankton becomes more homogeneous until the next season is ushered in. 9. Different genera of phytoplankters reach peaks of relative abundance at different times of year (e.g., Chaetoceros during the upwelling period, Rhizosolenia during the Davidson Current period), and it appears that some genera may be indicators of the initial stages of upwelling or of influxes of oceanic surface water. During the Davidson Current period of 1958-59 the predominance of tropical forms indicates considerable transport of offshore southern water into Monterey Bay. 10. There is no clear indication that total plankton volumes were influenced by the long-term temperature trend, but some of the individual genera, especially Ceratium, show a greater predominance during the warm years. 11. The results obtained in the present studies of Monterey Bay and environs coincide with findings in other areas of the coast well enough to indicate the site is suitable for continued monitoring of oceanographic conditions.