The changing structure of coastal communities in the Northern Gulf of Mexico

Abstract

Understanding how biodiversity and community structure in an ecosystem change over time is important in assessing the health of ecosystems and their preservation. This thesis explores the changes in marine macrobenthic community structure occurring in the northern Gulf of Mexico (nGOM) from 1987 - 2007 and relates these changes to abiotic factors. The primary questions asked in this thesis are: Has macrobenthic community structure changed in the nGOM? If so, then has community structure changed differently among localized areas, and are changes related to environmental conditions? Chapter One introduces the background and reviews the literature concerning the major concepts in the thesis. In Chapter Two, the nGOM region as a single ecosystem was studied to explore how the community changed between 1987 and 2007. Temperature in fall (autumn) rose significantly throughout the region, although there were no other significant changes found in temperature, dissolved oxygen, or salinity in either fall or summer. There were clear changes to the species which made up the community; however overall biodiversity was not observed to change. However, traits of species within the community were seen to change. Key changes to species showed that fish species that were more vulnerable to fishing, preferred warmer water, tolerated brackish water, preferred demersal and pelagic habitat and were of higher commercial importance increased in abundance. Fish species that declined in abundance were less vulnerable to fishing, preferred cooler water, reef-associated, marine-only and of less commercial importance than those that increased in abundance. In Chapter Three, localized environmental conditions and community measures as well as the correlations between them were mapped using novel mapping techniques to show spatial changes over time in a compact and easily visualized way. The analyses showed that there were significant localized shifts in biodiversity indices. Ecological measures (phylogenetic diversity, resilience, trophic level and vulnerability) the abundance and richness of species in different categories (taxonomic group, climate, habitat and salinity preference) throughout the nGOM, were correlated with different environmental changes. Significant localized temperature increases over time occurred throughout the region except for southern Texas, with the largest area of change seen off Mississippi in fall. Significant declines in dissolved oxygen (DO) were seen throughout the region, particularly in fall, except for the area near Louisiana, where the largest area of declining DO was in summer. Small areas of increased salinity were seen throughout the nGOM in fall and summer. In Southern Texas, overall richness increased and marine-only, demersal and tropical species were the main species groups that increased. Increases in biodiversity were correlated with higher temperatures and lower DO. Northern Texas also increased in species richness and demersal species abundance; however brackish-tolerant species also became more abundant. There was a drop in mean trophic level of individuals in this area. The Louisiana Delta area, which is strongly influenced by the Mississippi River, showed a strong shift from marine-only to brackish-tolerant species and an increase in the mean vulnerability of species. In the Mississippi area, tropical, temperate, marine-only and reef-associated species suffered the strongest declines, and these were correlated with increased temperature. Chapter four summarises the key findings of the thesis and discusses the major assumption in studying long-term data sets, potential future work and concludes the thesis. These findings show that communities in the nGOM have changed significantly over the study period and there are significant relationships between many community measurements and environmental shifts. This study has developed novel techniques to examine large sets of community data both temporally and spatially to find trends that have otherwise not been discerned. Overall the findings of this work demonstrate the effectiveness and usefulness of these unique techniques for elucidating trends in community structure throughout the region.

Date of Award	2015
Original language	English