Particulate matter production and consumption in deep mixed layers: observations in a warm-core ring

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Particulate matter variability is described in the contect of biological and physical processes in the core waters of WCR 82B between February and late June 1982. WCR 82B formed in mid-February with a probable mixed layer depth of 50 m. A series of heat loss and convective mixing events deepened the mixed layer to >300 m by March and to 400 m by early April 1982. Periods of convective mixing and transient stratification in the deep mixed layer were indicated by chlorophyll, nutrient, and temperature data collected during the last 10 days of April. Seasonal stratification was established by early May, and the core waters of WCR 82B became strongly stratified shallower than 40 m by June. Particulate matter samples were collected from the upper 1000 m by large volume in situ filtration in April and June 1982. Vertical distributions were obtained for particulate dry weight, organic carbon, calcium, biogenic silicon, and phosphorus as well as abundances of >1 mm size fecal matter and fecal pellets. The data show that particle production exceeded particle consumption by zooplankton in the euphotic zone from February to April. However, the particle concentration remained nearly constant in the euphotic zone, but increased between 50 and 400 m during this period. These observations suggest that mixed layer convection in March and April removed a significant fraction of particles from the euphotic zone into the deep thermostad. Analysis of the data shows that 67% of primary produced carbon was mixed into the thermostad during this time. Such conditions were favorable for the growth of herbivorous zooplankton which were dispersed several hundred meters below the surface. Comparisons of the standing stock of particles present in the upper 400 m in late April with supply rates of material due to down mixing suggest that the particle populations in this zone turn over on time scales of 10 days. After onset of seasonal stratification in early May, the down-mixed supply of newly produced carbon to deep thermostad waters was eliminated. Because the rates of euphotic zone particle production and loss were no longer balanced, a bloom of phytoplankton peaking in mid-May may have occurred. Observations showed that particles were removed from the thermostad between April and June, and calculations suggest that this loss (by zooplankton consumption) may have occurred in as short a time as one week following seasonal stratification. Following the removal of utilizable particulate matter, zooplankton shoaled to become concentrated in the upper 50 m by June. The continued high rate of primary production and the strongly increased zooplankton biomass in the upper 50 m in June resulted in enhanced production of fecal material in the upper 40 m. The high rates of zooplankton grazing in June resulted in the removal of most aggregate material by 110 m. These observations demonstrate that the coupling of physical, biological and chemical processes in the upper ocean occurs on time scales as short as 10 days. They also show that particulate matter is a sensitive indicator of the balance between production and removal processes in the upper 1000 m. Our data suggest that (1) zooplankton consumption as opposed to dark respiration is the dominant loss mechanism for phytoplankton carbon mixed below the euphotic zone into deep mixed layers, and (2) the imbalance between production and removal processes in the euphotic zone at the time of stratification caused by the cessation of mixing to depths below the euphotic zone leads to the development of the spring phytoplankton bloom.