The pelagic nitrogen cycle in a high CO2 world: What do we know so far?

Chair: Andrew McMinn

Nicola Wannicke (1) & Maren Voss (1)
1) Leibniz Institute for Baltic Sea Research (IOW), Department of Biological Oceanography, Seestrasse 15, D-18119 Rostock, Germany, Nicola.Wannicke@io-warnemuende.de

Background
The increase in atmospheric CO 2 has already caused significantly higher aquatic CO2 concentrations and lower pH values, changes which will continue over the next decades. Phytoplankton and other pelagic microbes will be widely affected (e.g. Liu et. al. 2010), and thus elemental cycles like the nitrogen cycle in the pelagial.

Methods
This review summarizes findings from six years research of the German BMBF funded project BIOACID with a focus on nitrogen fixation, combined with a literature survey of other nitrogen processes.

Findings
A strong stimulation of nitrogen fixation in tropical species like Trichodesmium and Crocosphaera has been reported (e.g. Levitan et. al. 2007, Hutchins et. al. 2013). However, for temperate brackish species like e.g. Nodularia and Dolichospermum either no significant effect was detected (Karlberg & Angela Wulff 2013, Brutemark et. al. 2015), or a stimulation during an exponential growth phase in the laboratory (Wannicke et. al. 2012) and even a negative impact (Czerny et. al. 2009). As an explanation for the observed differences, the availability of inorganic phosphorous and the physiological status of the bloom forming species will be discussed.

In contrast, nitrate assimilation seems not to be affected by elevated CO 2 (Clark et. al. 2014).

Nitrification on the other hand, appears to be more sensitive and decreasing at high CO 2 (Beman et. al. 2011). If true, the available form of dissolved nitrogen might be shifted from nitrate towards ammonium in the future ocean. Coastal sites, however, showed contrasting results with a high tolerance of nitrification against high CO2 concentrations (Fulweiler et. al. 2011). Potentially underlying reasons of these differences will be discussed.

Conclusions
For changes in the nitrogen cycle of the future ocean there are more conflicting than consistent results which we will try to consolidate.