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Introduction to the project
Introduction to the project
My second project is part of a master thesis of Pamela Lattanzi on the species richness, composition, and identification of Hydrozoans from a Chilean fjord.
Background information on the location
Background information on the location
The south of South America where the Chilean fjord region is situated is an important transition region. It connects the Antarctic with both the Atlantic and Pacific temperate oceans in terms of climate, biogeography, and biological conditions. Here we studied the Hydrozoan community since Hydrozoans are important in the coupling between benthic and pelagic systems and are very abundant in the Chilean fjord region. The fjord has varying hydrological (inputs, outputs, velocity) and bathymetrical characteristics geographically. It is characterized by high turbulence due to the river inputs as well as the high organic content resulting in a very low solar irradiance penetration. Even more, only 1% of the light reaches 13m. Dissolved oxygen also varies considerably with depth with rapidly decreasing trends starting from 20 m depth (where water is saturated). Those characteristics determine the Hydrozoan species able to survive in these locations.
Stratification
Stratification
One very important phenomenon observed in this region that possibly plays a big role in Hydrozoan composition is stratification. Stratification in these regions consists of a dominating halocline (= very rapid change in water salinity when going deeper) as well as a thermocline (= very rapid change in water temperature when going deeper). Brackish water (salinity < 31) originating from heavy rainfall combined with meltwater of glaciers forms a less dense top layer. This less dense layer becomes even less dense when it heats up in the summer since warmer water has a lower density than colder water. Deeper down, the Sub-Antarctic Waters (salinity > 33) are present. These are brought to the surface in upwelling systems by the eastwards blowing winds. In between, those layers mix, and modified sub-antarctic water of salinity between 31 and 33 is created.
The extent of the top freshest and warmest layer varies seasonally and with the tidal cycle from 4 to 10 m deep.
Next to temporal variations, spatial variations in water characteristics exist due to the variability in wind characteristics resulting in the strongest stratification in the north during spring, but vice versa in summer. Schneider et al (2014) also found that in the Puyuhuapi fjord, the water is permanently stratified in contrast with the Seno Magdalena fjord where waters are mixed.
Figures| Vertical profiles of the Puyuhuapi fjord in November (Schneider et al., 2014) . Both salinity (on the left) and temperature (on the right) profiles show a stratification with a halocline (left) and thermocline (right) around 5 m. These profiles are from the end of autumn and stratification will only be stronger in summer.
Samples
Samples
Samples were taken by SCUBA divers in the Puyuhuapi Channel and Seno Magdalena fjord in Chile whereafter they were preserved in formaline.
Sample analysis
Sample analysis
Pamela already analyzed a big part of the samples so I helped her to identify the rest as well as rechecking all the samples she did. This was done by looking at the sample under the stereomicroscope and using the literature to identify the found specimen. We also made permanent slides we could rewatch to compare specimens found in the sample with the previously identified species on the permanent slide.
Results
Results
Identified species
Identified species
Chilean hydrozoa.pptxData analysis:
Does Hydrozoan species composition depends on geographical location in the fjord and/or on depth?
Exploring with NMDS and cluster analysis
Before starting the statistical tests, I performed an NMDS on all the samples to see if they look grouped based on depth or site which was not the case. Furthermore, the cluster analysis also clustered samples randomly. This led me to suspect that I wouldn't obtain statistically significant results with the tests.
The best methods for calculating distances for binary data with a lot of zero's using this database is using Jaccard distances combined with an 'average' linking method which resulted in a correlation between cophenetic distance and the original distance of 85%.
Statistical test: PERMANOVA in PAST
In order to perform a PERMONOVA trustfully, the design has to be balanced, which means that there should be an equal amount of samples in each group (either site or depth). This was not the case when I used R since R requires me to remove the rows that consist of only zero's. These zero rows however are very important observations since these are not locations that were not sampled, but divers could not find any Hydrozoan at these depths and sites. The free software PAST was the solution.
PERMANOVA using PAST gave a significant influence of site on species compositions (F = 1.31, p = 0.02). However, posthoc pairwise comparisons Bonferroni corrected did not give any significant difference between sites. Depth also has a significant influence on species composition (F = 1.74, p < 0.0001) with only a significant difference between 5 and 15m (p = 0.012) when the p-values are Bonferroni corrected.
One note is that PAST was not able to test the second assumption of a PERMANOVA which is an equal dispersion of the samples among groups (depths or sites).
We hypothesized that there would be a difference in species composition in the top layers (5 to 10m) compared to the deeper layers due to stratification. Indeed, only in station PUY 26, Hydrozoans were present at 5 m. Hydrozoans need to be adapted to fresher water in order to be able to survive there. A Kruskal-Wallis test also indicates that there is a difference in species richness depending on depth (Chi² = 16.33, p = 0.005) with significant differences between 5-15 m (p = 0.005) and 5-20 m (p = 0.02).
My experience
My experience
It was so amazing to get to know so many new species of Hydrozoans. Pamela helped me big time by making the PowerPoint with the species she had already found so that the recognition process went faster. I was amazed at how quick I could recognize the different species. The discussions we had about identifications were also very nice and most of the time we agreed in the end on a species.
Schneider, W., Pérez-Santos, I., Ross, L., Bravo, L., Seguel, R. and Hernández, F. (2014). On the hydrography of Puyuhuapi Channel, Chilean Patagonia. Progress in Oceanography, [online] 129, pp.8–18. Available at: https://doi.org/10.1016/j.pocean.2014.03.007.
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