No abstract

In 1996 three cruises were conducted to simultaneously quantify the fine-scale optical and physical structure of the water column. Data from 120 profiles were used to investigate the temporal occurrence and spatial distribution of thin layers of phytoplankton as they relate to variations in physical processes. Thin layers ranged in thickness from a few centimeters to a few meters. They may extend horizontally for kilometers and persist for days. Thin layers are a recurring feature in the marine environment; they were observed and measured in 54% of our profiles. Physical processes are important in the temporal and spatial distribution of thin layers. Thin layer depth was closely associated with depth and strength of the pycnocline. Over 71% of all thin layers were located at the base of, or within, the pycnocline. The strong statistical relationships between thin layers and physical structure indicate that we cannot understand thin layer dynamics without understanding both local circulation patterns and regional physical forcing.

Samples from two coastal experimental ecosystems were incubated in vitro and sampled over 24 h. Production rates were measured by the 14C method, the O2 and CO, light-dark bottle methods, and the I80 method. 0, production in the experimental enclosures (volume -1.3 x lo4 liters) was also measured directly.Photosynthetic and respiratory quotients were close to 1 .O. Gross production values determined by 0, light-dark experiments, CO2 light-dark experiments, and I80 were similar. 14C production ranged from 60 to 100% of gross production measured in CO, light-dark experiments, indicating that 14C uptake is not precisely fixed with respect to other measures of community metabolism. There was no evidence that 14C or any other method underestimated the rate of primary production in vitro by more than 40%. Productivities in vitro ranged from 35 to 100% of those in the mesocosm at similar light intensities.In samples from one of the ecosystems, the rate of respiration in the light (calculated from I80 data) was an order of magnitude greater than the rate in the dark. This difference may be ascribed to either photorespiration or light enhancement of mitochondrial respiration.Turnover of microplankton populations in the ocean occurs on time scales of hours to days. Measurements of community turnover rates must be carried out with in vitro incubations, presenting two problems. First, it can never be claimed that processes oc-I

Differences between the carbon isotopic values of carbonates secreted by planktic and benthic organisms did not recover to stable preextinction levels for more than 3 million years after the Cretaceous-Tertiary mass extinction. These decreased differences may have resulted from a smaller proportion of marine biological production sinking to deep water in the postextinction ocean. Under this hypothesis, marine production may have recovered shortly after the mass extinction, but the structure of the open-ocean ecosystem did not fully recover for more than 3 million years.