Freeze-lysable inorganic nutrients in intertidal sediments: dependence on microphytobenthos abundance

Abstract

Freezing is a common treatment for the preservation of sediment samples. To test the role of microphytobenthos (MPB) abundance in the release of intracellular nutrients to the pore water due to cell breakage after freezing, referred to as freeze-lysable inorganic nutrients (FL-IN), parallel extractions were carried out from intertidal sediment cores collected in winter and summer from Cádiz Bay. After the determination of net production and dark respiration rates with O2 microsensors, sediment cores were subcored and sliced into several layers. The samples were divided into 2 fractions; the first was centrifuged to extract pore water (fresh) and the other was frozen at –80°C, thawed and centrifuged to extract pore water after freezing. NO2–, NO3–, NH4+ and PO43– were measured in the pore water extracted by both procedures. Chlorophylls a and c were extracted from the same sediment fractions. Freezing produced a significant increase in the pore water concentrations of all inorganic nutrients. Therefore, if the variable of interest is the inorganic nutrient concentration, pore water should be extracted from fresh samples. In addition, FL-IN correlated significantly with chlorophylls a and c (p < 0.01), r2 ranged from 0.54 for NO3– to 0.94 for NH4+, indicating that most FL-IN were released from MPB biomass. The relationships between chlorophyll a and FL-IN in winter and summer were significantly different, suggesting that nutrient accumulation by MPB changes seasonally and might affect nutrient cycling in intertidal sediments.