Invertebrate biodiversity in cold groundwater fissures in Iceland

Abstract

Iceland has an abundance of fissures that are parallel to the Mid-Atlantic Ridge where bedrock cracks as a result of continental rifting. Some fissures penetrate the aquifer and expose the groundwater within the bedrock, becoming springs. As such, groundwater fissures have uniform and constant physical and chemical environment but they can differ greatly in morphology. In addition, there is often great variation in depth within fissures and substrate types contrast between vertical rock wall and more heterogenous horizontal bottom. The variation in morphological environment may create dissimilar habitats with unique characteristics and/or influence distribution of resources. Our objective was to study macrozoobenthos communities in cold groundwater fissures in Iceland in relation to physical habitat by comparing invertebrate diversity and density both between fissures with different morphological characteristics as well as between substrate types and depths within fissures. Samples were collected in two fissures in SW Iceland, Silfra and Flosagjá. Assemblages were similar between fissures except for higher densities of cladocerans in Flosagjá fissure. Within fissures, there was significant difference in Shannon diversity between substrate types in Flosagjá, and ostracods were found in significantly higher densities on the bottom. The distribution of all other taxa groups was homogenous in both fissures regardless of depth gradient and substrate. Invertebrates were found to be living within and around a biofilm that covered the entire substrate. These biofilm mats are made from Cyanobacteria and benthic diatoms, which are successful under low light conditions and may minimize any effect of the heterogeneous habitat creating a uniform and suitable microhabitat for invertebrates regardless of depth and substrate type.

Keywords: biofilm; macrozoobenthos; species assemblages; spring; substrate.

Figure 1

A map of Lake Þingvallavatn in SV‐Iceland and the two groundwater fissures studied, Flosagjá (a) and Silfra (b). The approximate location of the two fissures is defined by a black rectangle. The location and number of transects within fissures are specified with black lines

Figure 2

Photograph of a diver in Flosagjá fissure in Þingvellir National Park SW Iceland. Photograph by Gísli Arnar Guðmundsson

Figure 3

Benthic biofilm covers the substrate in fissures. Photograph by Gísli Arnar Guðmundsson from Flosagjá fissure

Figure 4

The most common species found in fissures Flosagjá and Silfra, the chironomid Diamesa zernyi

Figure 5

Shannon diversity for Flosagjá and Silfra with range of values calculated from all sampling stations, mean (black dot) and median (black line). Whiskers reflect variability beyond the upper and lower quartile; the 3rd (Q3) and 1st (Q1) quartiles are demarcated by top and bottom of boxes. Empty circles are outliers. Result from a Wilcoxon signed‐rank test (p < 0.05) comparing Shannon diversity index between fissures is found in the lower right corner

Figure 6

PCA ordination diagram of all taxa (in italics) and sampling sites within Flosagjá (white circles) and Silfra (gray circles)

Figure 7

Shannon diversity index for invertebrate communities on different substrate types (Rock wall and bottom) in Flosagjá and Silfra with mean (black dot) and median (black line). Whiskers show variability beyond the upper and lower quartile; the 3rd (Q3) and 1st (Q1) quartiles are demarcated by top and bottom of boxes. Empty circles are outliers. Results from Wilcoxon signed‐rank test (p < 0.05) comparing Shannon diversity index between substrate types are shown for each fissure

Figure 8

Shannon diversity index for invertebrate communities at different depth on bottom in Flosagjá and Silfra with mean (black dot) and median (black line). Whiskers show variability beyond the upper and lower quartile; the 3rd (Q3) and 1st (Q1) quartiles are demarcated by top and bottom of boxes. Empty circles are outliers. Results from Spearman rho test (p < 0.05) comparing Shannon diversity index between depths are shown in the bottom right corner for each fissure

Figure 9

(a‐b) RDA ordination diagram of taxa (in italics with black arrows) and environmental variables (Substrate and Depth with gray arrows) in Flosagjá (a) and Silfra (b)