Linda Gilpin

The relationship between carbon and biovolume in marine microbial mesocosms under different nutrient regimes

Nutrient manipulation experiments were conducted on a natural planktonic community in outdoor mesocosms. Inorganic nitrogen (N) and silicon (Si) were added to achieve N:Si ratios of 1:1 and 4:1. Total particulate carbon (PC) biomass of the microbial assemblage was determined by elemental analysis. Cell volume measurements by microscope on individual components of the community (bacteria, diatoms, photosynthetic nanoflagellates, heterotrophic nanoflagellates, dinoflagellates and ciliates) were also made. We applied published C:volume relationships to determine the volume estimated C content (CBV) of these microbial groups and hence of the total assemblage. The total CBV and total PC were compared to test the applicability of C:volume relationships under different nutrient regimes both before and after nutrient exhaustion. For initial N:Si ratios of 1:1, prior to nutrient exhaustion, the relationship between CBV and PC was linear with a gradient of approximately 1, (0.99 ± 0.06), indicating that the published C:volume relationships accurately predicted the C content of the microbial assemblage. For N:Si ratios of 4:1, a linear relationship was again evident between CBV and PC (slope: 1.36 ± 0.08). However, statistical comparison using a general linear model indicated that the gradient of this relationship differed significantly from that when the N:Si ratio was 1:1, and hence CBV overestimated elemental C. For both N:Si ratios, subsequent to nutrient exhaustion (N or Si), and hence when the diatom fraction of the microbial assemblage was in yield-limited post-exponential phase, the two measures of biomass were not well correlated. This indicated that measured cytoplasmic cell volume was a poor indicator of C biomass within the microbial assemblage in nutrient-deplete conditions.

Tracking changes to a microplankton community in a North Atlantic sea loch using the microplankton index PI(mp)

&NA; Microplankton plays a vital part in marine ecosystems, and its importance has been recognized by the inclusion of microplankton community composition in regulatory frameworks such as the European Water Framework Directive and the Marine Strategy Framework Directive as an indicator of ecological status. Quantitative techniques are therefore required to assess the environmental status of the microplankton in a water body. Here we demonstrate the use of a method known as the microplankton index PI(mp) to evaluate changes in the microplankton community of the west coast Scottish Sea Loch Creran. Microplankton in this fjord has been studied since the 1970s, providing a data set spanning four decades. Our analysis compares an arbitrarily chosen reference period between 1979 and 1981 with a period between 2011 and 2013 and demonstrates that between these two periods community structure has changed considerably with a substantial drop in the numbers of observed diatoms accompanied by a rise in the number of autotrophic/mixotrophic dinoflagellates as well as an increase in the potentially toxin producing genus Pseudo‐nitzschia and that these are related to changes in both the intensity and timing of local patterns of precipitation. The PI(mp) is shown to be a useful and robust method to visualize and quantify changes in the underlying structure of the microplankton community and is a powerful addition to the toolbox of techniques needed to determine the health of our seas.

Biomass and productivity of seagrasses in Africa

Abstract There is growing interest in carbon stocks and flows in seagrass ecosystems, but recent global reviews suggest a paucity of studies from Africa. This paper reviews work on seagrass productivity, biomass and sediment carbon in Africa. Most work was conducted in East Africa with a major geographical gap in West Africa. The mean above-ground, below-ground and total biomasses from all studies were 174.4, 474.6 and 514 g DW m-2, respectively with a global range of 461–738 g DW m-2. Mean annual production rate was 913 g DW m-2 year-1 (global range 816–1012 g DW m-2 year-1). No studies were found giving sediment organic carbon, demonstrating a major gap in seagrass blue carbon work. Given the small numbers of relevant papers and the large geographical areas left undescribed in Africa, any conclusions remain tentative and much remains to be done on seagrass studies in Africa.

Carbon storage in the seagrass meadows of Gazi Bay, Kenya

Vegetated marine habitats are globally important carbon sinks, making a significant contribution towards mitigating climate change, and they provide a wide range of other ecosystem services. However, large gaps in knowledge remain, particularly for seagrass meadows in Africa. The present study estimated biomass and sediment organic carbon (Corg) stocks of four dominant seagrass species in Gazi Bay, Kenya. It compared sediment Corg between seagrass areas in vegetated and un-vegetated ‘controls’, using the naturally patchy occurence of seagrass at this site to test the impacts of seagrass growth on sediment Corg. It also explored relationships between the sediment and above-ground Corg, as well as between the total biomass and above-ground parameters. Sediment Corg was significantly different between species, range: 160.7–233.8 Mg C ha-1 (compared to the global range of 115.3 to 829.2 Mg C ha-1). Vegetated areas in all species had significantly higher sediment Corg compared with un-vegetated controls; the presence of seagrass increased Corg by 4–6 times. Biomass carbon differed significantly between species with means ranging between 4.8–7.1 Mg C ha-1 compared to the global range of 2.5–7.3 Mg C ha-1. To our knowledge, these are among the first results on seagrass sediment Corg to be reported from African seagrass beds; and contribute towards our understanding of the role of seagrass in global carbon dynamics.