Bioturbation and increasing temperature in shallow-lake sediments; is there an effect?

 Although Lakes only occupy 2% of the Earth’s surface (a meagre percentage when compared to the 70% covered by oceans); they still play substantial roles in global biogeochemical cycles. Shallow lakes are important carbon metabolism sites, with aerobic respiration being an important meahcniams in the carbon metabolism of lakes. Bioturbation is impacted by benthic animals, which re-work the sediment matrix; and their impact has been underestimated until now. Seasonal variation plays an important role in biological activity, and this experiment uses microcosm experiments to investigate how the impact of biurtabation on lake sediment respiration changes with increasing temperatures.

The bioturbator used for these microcosm experiments is Chironomidae larvae (Diptera), a species which dominates fresh waters and is considered highly important in aquatic bioturbation. Resazurin is a bioreactive tracer, and its decay is proportional to aerobic respiration within the system; thus it can be used to assess the temperature-dependent differences in sediment respiration. Resazurin is not susceptible to respiration of apneustic aquatic animals; therefore its use enables the independent quantification of sediment respiration impacts of chironomid bioturbation, from their own respiration.

Experiments used glass cylindrical mesocosms with a volume of 566ml; containing 200g of sediment from Lake Muggelsee (Berlin, Germany), with known nutrient levels. Bank filtrate water (250ml) was used in the mesocosm, and was constantly aerated to assure homogenous mixing and continuous oxic conditions in the water overlying the sediment. All animals used in the experiment were of similar ages and comparable sizes. They were used in 3 different densities which corresponded to in situ analysis of the source lake; using a range of 0, 1000 and 2000m^-2 in the experiment compared to 500- 2000 per m ² in situ.  Animals were acclimatised to each respective temperature (5, 10, 15, 20 and 30 ° C) 5 days prior to the experiment, and stable redox conditions were established in the sediment before commencing. Oxic interfaces were visible as ‘light-reddish brown’ patches in sediments and usually appeared between 24 and 36h, indicating that the 5-day long pre-experimental acclimatisation phase was sufficient. Four replicates (at each temperature) for each larval density were conducted. Aerobic respiration was quantified by measuring fluorescence of resazurin (because resazurin decay is proportional to aerobic respiration in the system (average r² =0.986), thus acting as a proxy for 0₂ consumption.

The microcosm experiments revealed respiration differences between bioturbated and non-bioturbated sediments, showing an increase of respiration with rising temperatures. At 5 °C, the difference in sediment respiration between bioturbated and non-bioturbated respiration was statistically not significant.  Whilst at 10°C and above, respiration differences between non-bioturbated and chironomid-bioturbated sediments were statistically significant, and increased with rising temperature. The largest difference was seen in the 30°C microcosm experiment, with respiration in microcosms containing 1000 larvae m^-2 being 4.4 x higher than in non-bioturbated sediments, and those containing 2000 larvae m^-2 exceeded non-bioturbated sediments by six times. Temperature dependent respiration was only highly significant in microcosms containing 1000-2000 larvae m^-2, and was not statistically significant in the non-bioturbated microcosms.

This paper, for the first time, has enabled the measurement of respiration not affected by the respiration of chironomids’; therefore showing that the increased respiration rates in this study, to be solely attributed by the bioirrigation-impacted sediment respiration. It was proven that increasing temperature significantly enhances the impact of chironomid bioturbation on sediment respiration (i.e.  Strong seasonal changes of sediment respiration sue to seasonal changes of lake). This paper shows that increasing global temperatures impact the sediment - microbial communities within lakes, and that even an increase of a few degrees may be sufficient to increase respiration rates within sediment. None the less, they still believe that more research is needed in this area; as already mentioned in the beginning - it is a heavily under explored area of microbiology.


I would highly recommend looking at the graphs in the paper, as they really help to explain the linear relationships between temperature and respiration. 

http://rsbl.royalsocietypublishing.org/content/roybiolett/12/8/20160448.full.pdf

Reviewed paper : Baranov, V., Lewandowski, J. and Krause, S., 2016. Bioturbation enhances the aerobic respiration of lake sediments in warming lakes. Biology Letters, 12(8), p.20160448.