Riverine sources of bioreactive macroelements and their impact on bacterioplankton metabolism in a recipient boreal estuary

Abstract

The loading of terrestrially derived macroelements, such as dissolved organic carbon (DOC), total nitrogen (N), and total phosphorus (P), to inland and estuarine waters is increasing in the northern hemisphere. These
macroelements often limit heterotrophic bacterioplankton metabolism (production and respiration), which in turn influence food web structures, exchange of greenhouse gases between the atmosphere and aquatic systems and oxygen consumption in estuarine ecosystems. In order to predict the impact of increasing macroelement inflows on the function and structure of boreal aquatic ecosystems it is vital to understand the potential sources of macroelements and their regulation of bacterioplankton metabolism. In this thesis, I aimed to determine whether terrestrially derived macroelement loadings to inland and estuarine waters can be utilized by bacterioplankton. In addition, I assessed how riverine influxes of DOC, N and P, alone or combined, regulate estuarine bacterioplankton metabolism. For this purpose, I applied a bioassay
approach to measure bacterioplankton respiration (BR) and production (BP) on samples from boreal freshwaters and estuarine systems. Terrestrially derived macroelement availability in brown-water lakes increased from DOC to N to P and averaged 2%, 31% and 49%, respectively. Although extremely low, relative
DOC bioreactivity increased with downstream transit for rivers with long water residence time and high fractions of urban and agricultural land use. Riverine bioreactive DOC was not the primary source of energy supporting bacterioplankton respiration at the Öre estuary, confirming the low bioreactive DOC exports expected for the Öre river. Instead riverine nutrients showed a more important effect on BR through stimulation of primary production, which in turn derived the organic carbon utilized by the bacterioplankton in the estuary.
However, BP at the Öre estuary was in general limited by P and BR by carbon. In light of expected increases in riverine loadings of DOC and P, and decreases of estuarine primary production, estuarine BP is expected to increase, as well as estuarine BR of riverine DOC inputs. These bacterioplankton responses may be
exacerbated by combined increases of riverine inflows of DOC, N and P. In summary, terrestrially derived macroelements can have a substantial impact on freshwater and estuarine bacterioplankton metabolism. Given the large role of bacterioplankton on the structure and function of aquatic systems, continuous changes in the input of terrestrial macroelements may have large consequences for boreal aquatic ecosystems.

Abstract (Swedish)

The loading of macroelements such as, dissolved organic carbon (DOC), total
nitrogen (N) and total phosphorus (P) from terrestrial soils to aquatic systems is
increasing in the northern hemisphere. This phenomenon has several ecological
and biogeochemical consequences for inland and coastal water systems, which are
linked to bacterial cycling of DOC, N and P. During growth, bacteria assimilate
macroelements into biomass (a process known as bacterial production), removing
DOC, N and P from the water column and channelling these elements to
consumers. Bacteria can also utilize organic carbon as a source of energy for
respiration. For this purpose they take up oxygen dissolved in the water and
release CO2 to the atmosphere. Increases in bacterial production have
consequences for aquatic food web structures, whereas increases in bacterial
respiration have implications for greenhouse gas emissions and for oxygen
concentrations of estuarine waters. Currently, it is still unclear the degree to which
land derived macroelements can be utilized by bacteria. Moreover, it is also not
understood how bacterial metabolism (production and respiration) will respond to
increased terrestrial macroelement fluxes.
In this thesis, I aimed to determine the fraction of the terrestrially derived
macroelement loading that can be utilized by bacteria. Moreover, I investigated
whether riverine inflows of DOC, N and P, alone or combined, limited the
metabolism of bacteria in estuarine waters. I found that on average only 2% of the
DOC exported from land was utilized by bacteria for production of biomass during
a seven-day period. This share amounted to approximately 50% in the case P,
which was thus the macroelement mostly available to bacteria among the three
macroelements studied. Yet, the relatively low DOC quality increased downstream
for rivers with long river water residence time and high catchment proportions of
agricultural and urban land. These riverine catchment features are thus important
to predict the export of oxygen consuming organic carbon (C) from rivers to
estuaries.
The DOC transported in the Öre river, was less important for support of
bacterial respiration than nutrients. These findings agree well with predictions of
DOC quality for the Öre river, based on the its catchment features. Riverine
nutrients stimulated primary production at the Öre estuary, which in turn supplied
organic carbon to bacteria. In general, bacterial production in the Öre estuary was
limited by P, while bacterial respiration was limited by organic C. In light of
predicted increases of riverine macroelement deliveries and expected reductions of
estuarine primary production, bacterial production will likely increase, as well as
the bacterial respiration of riverine delivered organic C. Both estuarine bacterial
production and respiration are expected to increase the most in response to
combined increases of riverine deliveries of DOC, N and P.
In summary, land derived macroelements can be substantially available to
freshwater and estuarine bacteria and impact their metabolism. Given the large
role of bacteria in food web structures and aquatic CO2 emissions, continued
changes in the input of terrestrial macroelements may have large implications for
boreal aquatic ecosystems.

• author: Soares, Ana
• supervisor: Martin Berggren; Emma Kritzberg; Andreas Persson
• opponent: Professor Seitzinger, Sybil, University of Victoria, Canada
• organization: Centre for Environmental and Climate Science (CEC); Dept of Physical Geography and Ecosystem Science; BECC: Biodiversity and Ecosystem services in a Changing Climate
• publishing date: 2018-02
• type: Thesis
• publication status: published
• subject: Natural Sciences
• keywords: Dissolved organic carbon, DOC bioreactivity, nutrient bioavailability, dissolved organic matter, bacterioplankton production, bacterial respiration, aquatic ecology
• pages: 110 pages
• defense location: Lecture hall “Världen”, Geocentrum I, Sölvegatan 10, Lund
• defense date: 2018-03-09 10:00:00
• ISBN: 978-91-7753-589-8
• ISBN: 978-91-7753-588-1
• language: English
• LU publication?: yes
• id: 411fcc85-c7b6-4570-a5c9-2a3b2957d9df
• date added to LUP: 2018-02-25 16:20:18
• date last changed: 2019-05-17 13:49:55