Abstract:
A bi-weekly seasonal study was conducted in Ashumet Pond (Cape Cod,
Massachusetts). The Redfield Ratio (106C:16N:1P) and alkaline phosphatase activity
(APA) were utilized in tandem as nutrient deficiency indicators (NDIs) for
phytoplankton. The study objective was to evaluate the limiting nutrient status of the
pond throughout the growing season.
The development of a high throughput method for fluorometrically measuring
APA allowed for a large quantity of pond-water samples to be analyzed. The new method
utilized a cytofluor, a fluorescence multi-well plate reader, which increased sample
throughput by 75% compared to a standard filter fluorometer method. The detection
limit, capability to measure APA at different time intervals, and performance at sea were
tested. APA measurements made using the cytofluor were comparable to those made
using a standard filter fluorometer, thus indicating that the cytofluor is a suitable and
preferred replacement to the fluorometer for APA measurements.
The presence of alkaline phosphatase, an inducible phospho-hydrolytic enzyme, is
commonly used as an NDI diagnostic for phosphate limitation. A nutrient enrichment
incubation re-affirmed the use of APA as a robust indicator of phosphate limitation in
phytoplankton. APA data indicate that the system experienced episodic periods of
phosphate-deficiency, implying that the limiting nutrient regime was not static, but was
changeable throughout the growing season. Seasonal trends in dissolved N:P and
particulate C:P ratios often contradict the APA results, however, suggesting that the
Redfield Ratio is an unreliable indicator of the overall nutrient limitation regime of the
pond. The observed discrepancies between C:N:P and APA can be reconciled by taking
into account seasonal changes in species composition, which played an important role in
driving seasonal APA trends.
