Humic matter and phytoplankton nutrient limitation in a changing environment

Abstract: Climate change leads to increased inputs of terrestrial dissolved organic carbon (DOC) and associated nutrients to freshwaters, potentially affecting the risk for eutrophication. While the effect of higher DOC concentrations on primary productivity (PP) due to light attenuation and increased competition by bacteria is rather well studied, the effect of DOC on PP due to effects on nutrient availability is still unclear. Therefore, laboratory incubation experiments with natural phytoplankton communities were performed to investigate the effect of DOC on phytoplankton growth under different phosphorus (P) and iron (Fe) regimes, under exclusion of light and grazing effects. The experiments were conducted with water originating from two sites with differing DOC character within Lake Mälaren, the third largest lake in Sweden. P (ambient, 50 μg L-1 added), Fe (ambient, 400 μg L-1 added) and DOC (low, ambient, high) concentrations were crossed in all possible combinations giving 12 different treatments. Chlorophyll a concentrations and initial and final chemical conditions were analysed after 7.5 days of incubation. Specific growth rates were by far highest in P addition treatments, showing that the phytoplankton community in Mälaren was mainly limited by P. Simultaneous addition of P and Fe further stimulated algae growth, indicating a co-limitation of Fe and P. The addition of Fe alone had no or a negative effect on growth rates, which might be explained by a strong binding of P to precipitated Fe. The effect of DOC on primary production depends on nutrient regime. Under ambient P conditions, DOC enhanced growth rates, probably due to a concurrent increase in nutrients associated to DOC. Under P-rich conditions, higher DOC concentrations resulted in lower growth rates compared to ambient DOC treatments. Fe showed a significant interaction effect with DOC under ambient P conditions. The direction of this effect depends on DOC quality, indicating that the different chemical composition of DOC influences Fe bioavailability differently. To determine which DOC type has a higher binding capacity for Fe, more research in primarily Fe-limited systems is needed. The study shows that PP in Lake Mälaren is mainly regulated by P and partly by Fe and DOC. Moreover it demonstrates that DOC affects the availability of Fe to phytoplankton. It is concluded that increased DOC concentrations result in higher primary productivity, but a lower sensitivity of phytoplankton to P inputs.