Geographic variation in coastal marine upwelling creates a complex seascape of environmental conditions that underlie biogeographic patterns of demographic rates, species distributions, and the structure of marine assemblages. Quantitative descriptions of spatial and temporal variation in assemblages can identify potential processes that regulate structure and dynamics of species at different scales. Understanding the spatial patterns of species demography and structure of assemblages across the seascape also informs more applied endeavors such as fisheries management, conservation of species diversity and species response to global change. This dissertation addresses patterns of community structure and demographic variation of populations in temperate kelp forest ecosystems and explores mechanisms that may generate these patterns.
In Chapter 1, I present one of the first comprehensive studies of kelp forest community structure across variable upwelling conditions. Variation in the strength and frequency of upwelling can influence coastal marine assemblages through bottom-up effects of temperature, nutrients, and larval delivery. However, high variability in community structure may exist at local scales due to species interactions, habitat structure, and other local-scale processes. I ask whether benthic species vary across regional scales (10s-100s km), reflecting variation in upwelling, or at local-scales (1-10s km), signaling possible effects of local processes. From replicate detailed surveys of kelp forest flora and fauna, I explicitly test the assumption that subtidal benthic communities exhibit variation in the abundance of dominant benthic functional groups associated with regional-scale variation in upwelling. I demonstrate that upwelling processes regulate biomass of functional groups in kelp forest assemblages, but that local-scale patterns are important for some species, highlighting the need to look at both aggregate and compositional aspects of natural assemblages.
In Chapters 2 and 3, I compare life history traits (i.e. growth, fecundity, and survival) among populations of Megastraea undosa, a benthic turbinid snail, across different upwelling regimes. Geographic variation in life history traits underlies patterns of population persistence, biogeography, evolutionary biology, and predicted response to global change and resource management; but, geographic variation in life history traits has received little attention, particularly in marine conservation and management.
Variation in coastal upwelling dynamics leads to differences in both temperature and food availability, two factors that can affect energy available for individual maintenance, reproduction and growth. However, temperature and food availability are negatively correlated in coastal marine systems dominated by upwelling and may lead to reduced energy available for multiple life history traits. In Chapter 2, I examine whether geographic variation in upwelling leads to trade-offs between growth and reproduction in Megastraea undosa, and whether geographic differences in trade-offs lead to differences in average individual fitness between populations. Using mark-recapture and histology techniques, I demonstrate that geographic variation in trade-offs between growth and reproduction exists and is related to variation in upwelling. In cold, nutrient-rich waters, higher growth rates lead to lower reproductive allocation, while in regions of less persistent upwelling, the opposite occurs. However, these two different life history strategies lead to similar lifetime cumulative reproductive output between regions. These results highlight the importance of considering multiple life history traits rather than single traits when examining geographic variation in population dynamics. Furthermore, I show that applying similar fisheries regulations across populations with varying demography may result in local depletion of populations.
In Chapter 3, I examine whether a relationship between seawater temperature and individual growth of M. undosa exists, and if this relationship varies with regional variation in upwelling conditions. I reconstruct time series of water temperature and growth of M. undosa using isotopes and growth marks in calcium carbonate opercula. Results indicate that M. undosa growth is correlated with water temperature, but the direction of the correlation depends on upwelling region. The mechanisms causing spatial variation in these relationships are likely the direct effects of temperature on metabolism coupled with the indirect responses of the dominant macroalgae to upwelling intensity and energy allocation to other vital rates.
|School Location:||United States -- California|
|Source:||DAI-B 70/03, Dissertation Abstracts International|
|Subjects:||Ecology, Biological oceanography|
|Keywords:||Coastal upwellings, Kelp forests, Marine communities, Rocky reefs|
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