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Stable isotopic insight into pelagic carbon cycling in Loch Lomond: a large, temperate latitude lake.

Bass, Adrian M (2008) Stable isotopic insight into pelagic carbon cycling in Loch Lomond: a large, temperate latitude lake. PhD thesis, University of Glasgow.

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Abstract

Lakes play an important role in biosphere carbon dynamics. Though proportionally they constitute a small surface feature on the planet, in many cases lakes are subject to significant subsidies of organic material from their catchments. This input of allochthonous organic material, in addition to autochthonous organic material, has shown that lakes, particularly in temperate and boreal zones, can be heterotrophic systems and as such are net producers of CO2. Thus, understanding the magnitude of fluxes of carbon through these limnetic systems is important if their contribution to ecosystem / global carbon dynamics is to be elucidated. In this research two separate field campaigns were undertaken with the goal of understanding if, and exactly how significant secondary (bacterial) production utilising allochthonous carbon is to overall pelagic production in Loch Lomond, Scotland. Stable isotopic composition of dissolved inorganic carbon (DIC), dissolved oxygen (DO), dissolved organic carbon (DOC) and total dissolved nitrogen (TDN), along with their respective concentrations, were measured in a temporal and spatial survey. Range in [DIC] and δ13CDIC was consistent with that predicted by the shifting balance between autotrophic and heterotrophic pathways. [DIC] peaked in the summer / autumn (0.27 ± 0.09 and 0.17 ± 0.05 mM, south and north basins respectively), reflecting a period when bacterial processing of allochthonous material is high, and thus so is CO2 production. This effect was more pronounced in the mesotrophic south basin of the lake, compared to the oligotrophic north. Surface waters in the south, middle and north basins were generally saturated in CO2 beyond atmospheric equilibrium and thus sources of CO2 to the atmosphere. δ13CDIC and δ18ODO exhibited seasonal and spatial variability, probably also a result of changing metabolic balance and inflow characteristics. Spring / summer peaks in δ13CDIC (-5.1‰ epilimnion maximum) are indicative of photosynthetic incorporation, and vice versa in the autumn / winter (-13‰ hypolimnion minimum) points towards respiratory dominance. δ18ODO is enriched during respiratory utilisation and peaks in the autumn / winter months. Depletion in δ13CDIC coupled to concurrent enrichment in δ18ODO observed with increasing depth (particularly during lake stratification) is assumed to again be a result of a shift in metabolic process dominance from autotrophic to heterotrophic (Myrbo and Shapley 2006). Spatial variability was consistent with the varying trophic states between basins, e.g., most enriched δ13CDIC was recorded in the more productive south basin compared to the middle or north. Dissolved organic carbon concentration also changed with position in the lake. Highest concentrations in the south basin were linked to a shallow gradient catchment, draining base rich soils and agricultural land, compared to the steep sloped, base-poor catchment in the north. The greater quantities of dissolved organic carbon in the south suggested that if bacterial processing of allochthonous material was significant it would likely be most prevalent in the south. During the spatial survey consistent and significant heterogeneity in DIC, DO and DOC was recorded. Although the same degree of variability may not be associated with other, more mophometrically / hydrologically simple lakes, this work has shown consideration of this possibility is advisable. The second field campaign used direct measurements of algal and bacterial productivity, using labelled stable isotope incorporation methods, to elucidate the balance between autotrophic and heterotrophic processes. Primary production (PP) followed a predictable seasonal pattern, peaking in the spring and remaining relatively high until autumn. During this period primary production generally exceeded bacterial production (BP) per litre. During the winter this pattern was reversed. Using integrated estimates of both PP and BP this work showed that BP exceeded PP in the pelagic zone for the majority of the year, and over much of the lake’s extent. Even in the epilimnion BP was regularly the more significant process through the water column, and thus it is concluded Loch Lomond is a heterotrophic system and a likely source of CO2 to the atmosphere. The PP: BP ratio ranged from 0.6 – 0.8 in the north basin, and 0.4 to 0.6 in the south. On average for the whole lake, bacterial production exceeded primary production by between 2,700 and 4,400 kg C day-1. In total it was estimated that PP processes approximately 970 tonnes of carbon per year and BP between 2,300 and 2,800 tonnes of carbon per year. The proportion of total pelagic production fuelled by bacterial utilisation of allochthonous carbon changed throughout the year. During peaks of PP in the spring and summer much of the bacterial carbon demand was met by autochthonous supply. During the autumn / winter allochthonous carbon utilisation dominated pelagic production and regularly contributed over 90% of total pelagic production. Combining estimated quantities of allochthonous carbon utilised in the north and south basins per m2 (the middle basin taken as an intermediate between the two) and combining it with GIS data on lake volume, the total quantity of terrestrially derived carbon processed in Loch Lomond was estimated at approximately 3,300 ± 2,100 kg Callo day-1. Both spatial and temporal surveys of natural abundance stable isotope ratios, along with concurrent measurements of algal and bacterial production, have provided substantial evidence for the importance of allochthonous carbon in Loch Lomond. Even minimum estimates imply a system dominated by bacterial production, fuelled by a proportionally high quantity of terrestrial material, thus producing excess CO2, and potentially fluxing CO2 to the atmosphere.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Dissloved Organic Carbon, Dissolved Inorganic Carbon, Dissolved Nitrogen, Allochthonous, Autochthonous, Bacterial production, primary production, microbial loop, pelagic production, loch lomond,
Subjects: Q Science > QL Zoology
Q Science > QD Chemistry
G Geography. Anthropology. Recreation > GE Environmental Sciences
Colleges/Schools: College of Medical Veterinary and Life Sciences
Supervisor's Name: Adams, Dr Colin and Waldron, Dr Susan and Preston, Professor Tom
Date of Award: July 2008
Depositing User: Mr Adrian M Bass
Unique ID: glathesis:2008-07-221
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 12 Nov 2008
Last Modified: 10 Dec 2012 13:17
URI: http://theses.gla.ac.uk/id/eprint/221

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