Wetlands and the presence of fish. In

Wetlands are the largest natural source of methane (CH4).
In this project we focus on different processes influencing CH4
emissions to the atmosphere in shallow waters. The magnitude of CH4
emissions is the resultant of the amount of CH4 produced and the
amount consumed in the system. Both processes are strongly influenced by oxygen
concentrations, which are in turn influenced by the degree of eutrophication,
the type of vegetation present in the system and the presence of fish. In a set
of complementary mesocosm experiments in China and in the Netherlands we will
assess the relative importance of these variables on: CH4
production, CH4 oxidation and the resulting CH4
emissions. With a
34 times higher warming potential than CO2, and a 2.5-fold increase
of its atmospheric concentrations since preindustrial times, methane (CH4)  has become the
second most important long-lived greenhouse gas. (Stocker et al. 2013) Following a period of stable
atmospheric CH4 levels in the period between 1999 and 2006, concentrations
are rising again since 2007, reaching 1853 ppb in 2016. (WMO 2017) Freshwaters and wetlands are the largest
natural source of CH4 , with a global emission rate of approximately
257 Tg CH4 yr-1. (Kirschke 2013) The intensity
of aquatic CH4 emissions, however, varies greatly among systems and
little is known about what drives these differences. The magnitude of CH4
emissions is the resultant of the amount of CH4 produced and the
amount consumed in the system. Both processes are strongly influenced by oxygen
concentrations, which are in turn influenced by the degree of eutrophication,
the type of vegetation present in the system and the presence of fish. The emission of CH4
from wetlands is a resultant of CH4 production and CH4
consumption (i.e. CH4 oxidation). Production of CH4
occurs largely in sediments and is strongly influenced by temperature (Marotta et al. 2014), substrate availability (MOLONGOSKI & KLUG 1980), and (King 1990). The process of oxidation is
performed by methanotrophs in (oxic) sediment layers, in the water column and
on submerged plants.

 

In shallow systems
there are three principle pathways for sediment-produced CH4 to
reach the atmosphere: 1) via ebullition (bubble flux), 2) via diffusion through
the water layer, 3) via aerenchym of vascular plants (the so-called “chimney
effect” (Bastviken et al. 2004)). Eutrophication, i.e. increased nutrient
loading (nitrogen and/or phosphorus), threatens wetlands around the world. It
increases the biomass of plants and fish and changes the lake’s community
composition (Moss 2011). It likely also influences CH4
production and oxidation. Methane production might be stimulated due to the
increased organic
matter production and
subsequent decomposition which rates strongly depend on the plant species. On
the other hand, eutrophication may also
enhance CH4
oxidation by stimulating methanotroph growth (Veraart et al. 2015) and though enhanced growth of the plants
thereby providing a larger niche for methanotrophs. Methane oxidation
rates strongly vary among plant species as well (Yoshida et al. 2014). Eutrophication enhanced biomass of plants
may, depending on the species, also promote CH4 emissions through the
increase of the “chimney” emission pathway. CH4 transported through
this pathway largely escapes oxidation. Benthic fish likely have a strong
effect on CH4 emissions
as well. Their bioturbation of the sediment leads to a higher oxygen
availability in the sediment which may reduce anoxic methanogenis and increase
CH4 oxidation. Fish disturb the sediment which may also prevent CH4
to build up in the sediment and thereby decrease ebullition. Bubbles of CH4
quickly ascent to the water surface thereby escape possible
oxidation. Methane diffusing toward the water surface is much more prone to be
oxidized before reaching the water surface. Fish may, thus enhance CH4
oxidation in the water column as well. In short, there are many interacting
effects either through stimulating or hampering CH4 production and oxidation. The
overall effect of eutrophication (either N and P loading), the presence of fish
and the occurring submerged plant species on the CH4 balance and
consequent CH4 emissions to the atmosphere are therefore unclear (see also
Moss et al. 2011). Insight into the
relationship between within-lake conditions and CH4 emissions will
allow us to better estimate regional CH4 emissions and possibly
contribute to reduction of CH4 emissions in shallow lakes through
management measures. In a set of mesocosm
experiments in China we assessed the relative importance of these variables on:
CH4 production, CH4 oxidation and the resulting CH4
emissions. We examined diffusion and ebullition rates.

The Chinese experiment focused on: nitrogen loading to
mimic 2 different degrees of nitrogen loading, submerged plant species (3
species per mesocosm) and fish (presence/absence) and submerged plant species
(3 species) resulting in 30 units.