Fall 2006 Update: Comparison of Spring and Fall Conditions

For most regions, seasonal production can be partitioned into to spring and fall blooms, thus making the contrast in conditions during these two periods important. As described previously, spring surface water temperatures have declined in recent years from a peak in temperature around the year 2000. In contrast, fall temperatures appear to be continuing to increase or maintain higher levels in the Gulf of Maine, Southern New England, and Mid-Atlantic Bight (see figure). Despite being represented by a shorter time series, chlorophyll concentrations have some well developed trends as seen in the fall data where chlorophyll has declined in recent years with exception of fall bloom in the Mid-Atlantic Bight area. Spring bloom trends are less well developed, but the data do suggest that most areas are either maintaining bloom levels or increasing in chlorophyll concentration, with the exception of the Gulf of Maine, where the spring bloom appears to be trending downward over the past decade (see figure). The importance of seasonal dynamics is seen in the comparison of fall and spring zooplankton biomasses. In our prior advisory we stated that 'Regional declines in fall zooplankton coupled with regional increases in annual zooplankton indicate important regional dynamics affect the shelf-wide annual patterns.' The observed increases in spring zooplankton are apparently large enough in magnitude to cancel the decreases in fall zooplankton and result in modest regional increases in the annual zooplankton biomasses (see figure). The obvious implication is that the decline in fall primary production is affecting zooplankton populations. The continued increase in spring zooplankton populations is not supported by a direct relationship with chlorophyll concentration, but may be attributed to a shift in zooplankton community structure.

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