Bacterioplankton nutrient limitation in the Öre estuary: constraints on winter carbon metabolism

Abstract

With estuarine bacterioplankton as a key player in the nutrient cycle and the amount of DOC transported to sea increasing, it is important to gain a better understanding of how the processes within the catchment and estuary control the degradation of DOC by bacterial communities. Future climate change scenarios add a new dimension to this question, with predicted precipitation increases translating into increased water flows at the catchment level and increased DOC transport to the sea. This will have a direct effect on the estuarine bacterial communities involved in nutrient recycling. The focus of this study was to investigate how C, N, P and salt concentrations interact in the regulation of bacterial DOC degradation rates. It was hypothesized that primary C-limitation would occur during low flow conditions for both bacterial production (BP) and bacterial respiration (BR) at the 1m estuarine depth, and N-limitation would occur at high flow conditions. Co-limitation between C-and-N, C-and-P, the seasonality of nutrient limitation at the 15m estuarine depth and the 1m sea depth were also investigated. Sampling was done in the Öre estuary, in northern Sweden between October 2014 and March 2015. A full factorial design was implemented, where C, N, P and salt were added to samples in all possible combinations. BR in the form of dissolved oxygen consumption was measured in a controlled setting and BP was measured using the 3H-leucine incorporation method. BP was C-limited during low winter flow and P-limited during the high flow event in late October 2015. BR was C-limited throughout the entire sampling period. Additionally, BP was predominantly co-limited by C and N during low flow periods and BR was mainly co-limited by P throughout the sampling period. BP and BR at the 15m estuarine depth and the sea 1m depth followed the same trends as the 1m estuarine depth, with the former two generally having greater magnitude of increases for both bacterial processes tested. In conclusion, all hypotheses were supported with the exception of P-limitation during high flow events, instead of the predicted N-limitation. C-limitation appears to be more generic than was initially expected, which reinforces the importance C plays in the nutrient cycle. This study clearly showed that increased DOC concentrations in the boreal regions strongly affected bacterioplankton processes especially during low flow, winter conditions. In the case where N and P concentrations increased as well, planktonic processes increased further as a result of synergistic interactions. Future studies should assess the magnitude of impact for potential hypoxic zones which could lead to fish kills in the area and also for higher CO2 emissions and what effects these would have on the estuarine and marine trophic chain overall.

Popular Abstract

Nutrient limitation and constraints on winter carbon metabolism in the Öre estuary

With increasing DOC transported to sea, it is important to gain a better understanding of how the processes within the catchment and estuary control the degradation of DOC by bacterial communities. Future climate change scenarios add a new dimension to this question, with predicted precipitation increases translating into increased water flows at the catchment level and increased DOC transport to the sea. This will have a direct effect on the estuarine bacterial communities involved in nutrient recycling.

Sampling was done in the Öre estuary, in northern Sweden between October 2014 and March 2015. A full factorial design was implemented, where C, N, P and salt were added to samples in all possible combinations. Bacterial respiration (BR) in the form of dissolved oxygen consumption was measured in a controlled setting and bacterial production (BP) was measured using the 3H-leucine incorporation method.

BP was C-limited during low winter flow and P-limited during the high flow event in late October 2015. BR was C-limited throughout the entire sampling period. Additionally, BP was predominantly co-limited by C and N during low flow periods and BR was mainly co-limited by P throughout the sampling period. BP and BR followed the same trends for both the deeper estuarine sampling site of 15m and the sea 1m site.

Study implications

Overall the constraints on winter C are relatively small and the bacterial processes will exhibit a quick and strong response to C pulses regardless of increased N and P concentrations. These trends are expected to become more prominent with increasing climate changes. As such we require a still better understanding of how the processes within the catchment and estuary control degradation of DOC by bacterial communities and how these will vary seasonally.

This further shows that C-limitation is more commonplace than initially thought and mirrors increasing literature reports for the same trend. An increase in DOC concentrations will therefore stimulate the bacterial processes throughout estuaries and will likely increase the organic matter reaching the sea. As a result, oxygen consumption at all estuarine depths will also increase, potentially leading to anoxic patches or increased CO2 emissions to the atmosphere.

Supervisor: Martin Berggren, Department of Physical Geography and Ecosystem Science
Master Degree Project 45 credits in Aquatic Ecology
Department of Biology, Lund University

• author: Custelcean, Ioana
• supervisor: Martin Berggren
• organization: Degree Projects in Biology
• course: BION01 20142
• year: 2015
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 8312216
• date added to LUP: 2015-12-15 09:14:13
• date last changed: 2015-12-15 09:14:13