Linking bacterial community structure to advection and environmental impact along a coast-fjord gradient of the Sognefjord, western Norway

Abstract

Here we present novel data on bacterial assemblages along a coast-fjord gradient in the Sognefjord, the deepest (1308 m) and longest (205 km) ice-free fjord in the world. Data were collected on two cruises, one in November 2012, and one in May 2013. Special focus was on the impact of advective processes and how these are reflected in the autochthonous and allochthonous fractions of the bacterial communities. Both in November and May bacterial community composition, determined by Automated Ribosomal Intergenic Spacer Analyses (ARISA), in the surface and intermediate water appeared to be highly related to bacterial communities originating from freshwater runoff and coastal water, whereas the sources in the basin water were mostly unknown. Additionally, the inner part of the Sognefjord was more influenced by side-fjords than the outer part, and changes in bacterial community structure along the coast-fjord gradient generally showed higher correlation with environmental variables than with geographic distances. High resolution model simulations indicated a surprisingly high degree of temporal and spatial variation in both current speed and direction. This led to a more episodic/discontinuous horizontal current pattern, with several vortices (10-20 km wide) being formed from time to time along the fjord. We conclude that during periods of strong wind forcing, advection led to allochthonous species being introduced to the surface and intermediate layers of the fjord, and also appeared to homogenize community composition in the basin water. We also expect vortices to be active mixing zones where inflowing bacterial populations on the southern side of the fjord are mixed with the outflowing populations on the northern side. On average, retention time of the fjord water was sufficient for bacterial communities to be established.

Keywords: ARISA; Bacterial community composition; Environmental impact; Fjord current modelling; Fjord microbiology; Microbial biogeography; Stratified water masses.

Fig. 1

Map of the Sognefjord with sampling locations marked. Station 1a was sampled only in May 2013, and station 1b in November 2012 only.

Fig. 2

Profile of the approximate bathymetry of the Sognefjord, with water masses and major currents displayed. BSW: Brackish Surface Water, CF: Compensational flow and CW: Coastal water.

Fig. 3

Contour plots of Temperature, Salinity, Oxygen, NO₃ and chl a fluorescence measured in November 2012 and May 2013. Sampling stations are indicated by arrowheads and numbers or filled circles. On unmarked stations only CTD profiles were taken. The upper part of the plot indicates the upper 100 m of the fjord, whereas the lower part shows the entire water column. The 33 isohaline (marked in bold) indicates the transition between the brackish and intermediate layer.

Fig. 4

(a) Time series of the along-fjord current component m s⁻¹ in the upper 10 m and the upper 200 m respectively at the location St. 4 in the period November 1–20, 2012, from the numerical model results. Positive (red) and negative (blue) colors denote flow out of and into the fjord, respectively. (b) Vertical section along the southern side of the Sognefjord of the mean along-fjord current component [m/s] in the period November 1–20, 2012, from the numerical model results. The coast is to the left and the fjord head to the right. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 5

Time series of the wind speed m s⁻¹ for the Southerly component at the coast (blue line) and the along-fjord wind near St. 4 (red line) in the period November 1–20, 2012 (a) and for May 1–28, 2013 (b), from the numerical atmospheric model results. Positive values are towards the North or into the fjord. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 6

a) Time series of the along-fjord current component m s⁻¹ in the upper 10 m and 200 m respectively at the location St. 4 in the period May 1–28, 2013, from the numerical model results. Positive (red) and negative (blue) colors denote flow out of and into the fjord, respectively. (b) Vertical section along the southern side of the Sognefjord of the mean along-fjord current component m s⁻¹ in the period May 1–28, 2013, from the numerical model results. The coast is to the left and the fjord head to the right. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 7

Mean horizontal current m s⁻¹ for the surface (a) and at 20 m depth (b) in the period November 1–20, 2012.

Fig. 7

Mean horizontal current m s⁻¹ for the surface (a) and at 20 m depth (b) in the period November 1–20, 2012.

Fig. 8

Horizontal current m s⁻¹ at 20 m depth for respectively 80 h (a), 200 h (b) and 300 h (c) after November 1, 2012, illustrating episodes of inflow, calm and outflow of water in the Sognefjord according to the numerical current model.

Fig. 8

Horizontal current m s⁻¹ at 20 m depth for respectively 80 h (a), 200 h (b) and 300 h (c) after November 1, 2012, illustrating episodes of inflow, calm and outflow of water in the Sognefjord according to the numerical current model.

Fig. 8

Horizontal current m s⁻¹ at 20 m depth for respectively 80 h (a), 200 h (b) and 300 h (c) after November 1, 2012, illustrating episodes of inflow, calm and outflow of water in the Sognefjord according to the numerical current model.

Fig. 9

Distance-decay curves for the bacterial community in November (solid line) and May (dashed line) in the three different water layers, indicating bacterial community composition changes at different depths with increasing distance in the fjord. Calculations are based on the logarithm (ln) of community similarity calculated from 1 – Jaccard distance matrices.

Fig. 10

Illustration of how allochthonous bacterial communities enter the fjord in November 2012 (a–c) and May 2013 (d–f), and mixes with the resident bacterial community as estimated using Sourcetracker. Arrowheads indicate source samples. Each circle represents one sample, the size of the circle indicates how much (%) of the community that is influenced by the source communities. Samples marked green indicate freshwater influence from water located at the entrance to side-fjords. Yellow circles indicate Coastal water/Atlantic water influence from samples located outside the sill. Contours indicate salinity gradients in the fjord. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)