Abstract
A common and established view is that increased inputs of nutrients to the sea, for example via river flooding, will cause eutrophication and phytoplankton blooms in coastal areas. We here show that this concept may be questioned in certain scenarios. Climate change has been predicted to cause increased inflow of freshwater to coastal areas in northern Europe. River waters in these areas are often brown from the presence of high concentrations of allochthonous dissolved organic carbon (humic carbon), in addition to nitrogen and phosphorus. In this study we investigated whether increased inputs of humic carbon can change the structure and production of the pelagic food web in the recipient seawater. In a mesocosm experiment unfiltered seawater from the northern Baltic Sea was fertilized with inorganic nutrients and humic carbon (CNP), and only with inorganic nutrients (NP). The system responded differently to the humic carbon addition. In NP treatments bacterial, phytoplankton and zooplankton production increased and the systems turned net autotrophic, whereas the CNP-treatment only bacterial and zooplankton production increased driving the system to net heterotrophy. The size-structure of the food web showed large variations in the different treatments. In the enriched NP treatments the phytoplankton community was dominated by filamentous >20 µm algae, while in the CNP treatments the phytoplankton was dominated by picocyanobacteria <5 µm. Our results suggest that climate change scenarios, resulting in increased humic-rich river inflow, may counteract eutrophication in coastal waters, leading to a promotion of the microbial food web and other heterotrophic organisms, driving the recipient coastal waters to net-heterotrophy.
Highlights
The efficiency of pelagic food webs determines the amount of energy produced at the highest trophic levels and is highly dependent on the pathway through which energy and matter flows from basal trophic levels to the top predators [1]
Average bacterial growth rate was,30% higher when from the Ore River (FO) was added to the seawater than in the control (Fig. 1), this could not be statistically verified (p.0.05, ANOVA, and T9 post-hoc test, Spjøtvoll and Stoline, 1973)
The traditional view that nitrogen and phosphorus load will result in the eutrophication of recipient coastal waters (e.g. [33]) may be questioned in certain scenarios
Summary
The efficiency of pelagic food webs determines the amount of energy produced at the highest trophic levels and is highly dependent on the pathway through which energy and matter flows from basal trophic levels (phytoplankton and bacteria) to the top predators (e.g. fish) [1]. Phytoplankton are autotrophs which use inorganic carbon as their carbon source, while heterotrophic bacteria, utilize organic carbon from autochthonous or allochthonous sources. Both are osmotrophic organisms, competing for inorganic nutrients via diffusion into their cells. The relative importance of these functional groups differs between oligotrophic and eutrophic waters [2], and between autochthonous aquatic systems and those influenced by terrestrial run-off [3]. The importance of bacteria is relatively high in low productive areas influenced by allochthonous organic material, while in more productive and eutrophic areas phytoplankton dominate. Netheterotrophy has been found to be more pronounced in humicrich than in clear water systems [4], probably due to the dominance of bacteria which often contribute to the largest fraction of the respiration in pelagic systems [5]
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