Pisternick, Timo (2018) Community structure and abundance of reef fish in a lagoon characterized by freshwater and high nutrient input through submarine groundwater discharge. (Master thesis), University of Bremen, Bremen, 113 pp.

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Submarine inflow of freshwater from land into the ocean (fresh) submarine groundwater discharge (SGD) is increasingly recognized as an important source of local nutrient and pollutants influx to coastal ecosystems. Still, very little is known about the effects of SGD on ecosystem functioning. While there is evidence of a positive link between SGD derived nutrients and phytoplankton abundances, further research is needed to assess the effects of SGD on the productivity of higher trophic levels such as ichthyofaunal communities.
To follow up on this relationship, two sites in two tropical coastal lagoons (Trou aux Biches and Flic en Flac, Mauritius) were sampled during the summer months (October 2017 – December 2018). In Trou aux Biches the hydrology in the lagoon’s southern part is highly influenced by six distinct freshwater springs, whereas the lagoon’s northern part served as a control site. In Flic en Flac hydrology in the lagoon’s northern part is influenced by diffuse groundwater seepage, whereas the lagoon's southern part served as the control site. In each lagoon, abiotic parameters and nutrient concentrations were recorded. Additionally, the dry weights of total suspended solids (TSS) as a proxy for primary producer abundance, the compositions of benthic cover, fish abundances, as well as community structuring were recorded for Trou aux Biches,
Water nutrient (PO43-, SiO44-, NO3-, NO2-, NH3) levels were higher, whereas salinity and pH values were lower in the SGD affected parts when compared to the strictly marine parts of the lagoons. In Trou aux Biches, TSS and coral cover were significantly higher at the SGD site while macroalgae cover was higher at the control site. In addition, the SGD affected part of the lagoon exhibited significantly higher fish species richness and abundance when compared to the control site. Also, abundances of commercially important fish species directly at the groundwater springs were significantly. Non-metric dimensional scaling of the fish assemblage showed a partitioning of sites based on benthic characteristics influenced by SGD. Herbivorous and invertivorous fish species were identified by SIMPER analysis as main drivers of trends setting the freshwater springs and apart within the MDS plot.
Obtained results provide first evidence of a SGD driven positive relationship between high nutrient loadings, elevated primary production, and enhanced secondary consumer abundances in tropical coral reef lagoons. These findings will have implications for coastal management advice and secondary consumer productivity appraisal on tropical islands.

Document Type: Thesis supervision (Master thesis)
Thesis supervisor: Ekau, Werner and Moosdorf, Nils
Research affiliation: Ecology > Fisheries Biology
Biogeochemistry and Geology > Submarine Groundwater Discharge
Projects: SGD-NUT
Date Deposited: 08 Sep 2020 16:27
Last Modified: 01 Oct 2020 13:01
URI: http://cris.leibniz-zmt.de/id/eprint/4109

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