Impact of planktonic food web structure on nutrient retention and loss from a late summer pelagic system in the coastal northern Baltic Sea

Abstract

ABSTRACT. A large-scale mesocosm experiment was carrled out in the coastal area of the northern Balt~c Sea in order to study limiting (bottom-up) and controlling (top-down) factors of the late summer pelagic community. The experiment was conducted in 5 floating 30 m,' mesocosms manipulated with nutrient (N and P) enrichments and fish (stickleback fry) according to a cross-over experimental design with rotating treatments. Concentrations of particulate organic C, N, P, and chlorophyll a as well as the development of bacteria, phytoplankton, protozoa, and mesozooplankton biomass were followed for 21 d, and sedimentation was measured. Nutrient enrichments induced phytoplankton blooms with equal biomass peak levels In all mesocosms However, the t~m i n g of the enrichment and the effect of the top-down manipulation resulted in diversified structure of planktonic communities in each mesocosm. Basically 2 kinds of system emerged: (1) mesocosms that had received nutrients immediately after the start of the experiment developed towards more regenerating systems where both N and P were retained to greater extent; (2) mesocosms that received nutrients after a 5 d lag-period developed towards a 'new production' type of system. In the latter kind, accumulation and loss of N followed closely the development of autotrophic biomass. In all mesocosms, N-limitation was mainta~ned due to greater sedimentary loss of N , while P was retained more effectively with~n the detrital pool of the pelagic system. The cascading effect of top-down manipulation influenced the grazer community and resulted in a different functional response in each manipulated mesocosm. These results indicate that during the process of eutrophication, the food web structure, timing of the fertilization, and alternative grazinglpredation strategies of the planktonic heterotrophs have a crucial impact on the retention and loss of nutrients from the pelagic system.

Groundwater and nutrient discharge through karstic coastal springs (<i>Castelló</i>, Spain)

“…In karstic areas, P limitation may be driven by sequestration of P in calcareous sediments (Elser et al, 2007). Phosphorus has been reported as potentially having greater regulatory importance than nitrogen in several coastal areas (Bonin et al, 1989;Heiskanen et al, 1996;Mahoney, 1989). The estimated SGD associated fluxes of nutrients could indeed alter the nutrient equilibrium of the Badum biological community since coastal stations barely influenced by SGD (A7 in October 2006 and S16 in June 2007) presented Si:N:P ratios with higher N and P with respect to Si (i.e., 16:25:0.9 and 16:58:3.3, respectively) than all the other stations inside the fresher SGD plume (average of 16:22:0.1 in October and 16:23:0.3 in June).…”

Seasonal phytoplankton nutrient limitation patterns as revealed by bioassays over Baltic Sea gradients of salinity and eutrophication

“…Mechanisms for these shifts could include selective (non-Redfield) removal of nutrients from the euphotic zone via sedimentation (Heiskanen et al 1996, Thomas et al 1999, unbalanced losses of N and P at the sediment-water interface via denitrification and, according to oxygen conditions, iron precipitation or release of PO 4 (Seitzinger 1988, Blomqvist et al 2004. Seasonal and episodic mixing events introduce the modified (in terms of N:P) near-bottom waters back to the productive layers.…”

Predicted impacts of elevated temperature on the magnitude of the winter‐spring phytoplankton bloom in temperate coastal waters: A mesocosm study

“…Since the fate of the photosynthetically produced organic matter can significantly influence the trophic structure of marine systems, it is important to understand the relationship between temperature and bloom magnitude. In general, phytoplankton levels are controlled by a balance between ''bottom-up'' control through nutrient limitation and resource competition and ''top-down'' processes such as grazing (Ryther and Dunstan 1971;Schindler 1974;Carpenter et al 1988;Durbin et al 1992;Heiskanen et al 1996). The interactions between these controlling factors regulate the biomass levels of the phytoplankton throughout the annual cycle and during bloom periods.…”