Nitrate time-series May

Nitrate time-series August-Sept

The macro-nutrients Nitrate, Phosphate and Silicate fuel the growth of the phytoplankton community. Nutrients are replenished into surface waters each year by deep mixing of the water during the stormy winter months.

Cruises each May (left) occur around the period when the water column stratifies trapping phytoplankton cells in water with high light and (at least initially) high nutrients. Rapid growth of the phytoplankton consumes nutrients that were initially around 30-40 ┬ÁM Nitrate, converting it into phytoplankton biomass. Timing of stratification, and timing of the ensuing bloom produce large variability between years in when nutrient concentrations are depleted from surface waters (i.e. upper 10m). Generally nutrients are depleted earlier in warmer years and later in colder years, although there are numerous complications to this simple generalization. The warmer years of 2003-2006 contrast sharply the colder years of 2007-2009.

 

Cruises in August/September (left) show that macro-nutrient are literally absent from the surface water during the late summer. Strong temperature and salinity stratification of the water during summer prevents the large pool of deep nutrients from being mixed back into surface waters. The shift of late summer cruises from August to early September starting in 2005 has placed cruises near to the period were surface water temperatures have begun to cool slightly. Stormier weather In September then begins to mix surface waters slightly deeper, liberating some deeper nutrients back into surface waters where they may be exploited for a last pulse of phytoplankton growth.

Show patterns for Surface Phosphate & Silicate