Hauxwell Jennifer

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Hauxwell
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Jennifer
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  • Preprint
    Light dependence of Zostera marina annual growth dynamics in estuaries subject to different degrees of eutrophication
    ( 2005-05-20) Hauxwell, Jennifer ; Cebrian, Just ; Valiela, Ivan
    In temperate, shallow systems with clear waters the temporal dynamic of eelgrass (Zostera marina) growth is closely associated with the seasonality of irradiance at the water's surface. It has been recently suggested that increasing eutrophication, via light attenuation by increased algal growth, may disrupt the close temporal association between eelgrass growth and surface irradiance often found in pristine sites. Here, we test this hypothesis by examining the coupling between eelgrass growth dynamics and surface irradiance over an annual cycle in four shallow estuaries of the Waquoit Bay system (Massachusetts, USA) that have similar physical characteristics, but are subject to different land-derived nitrogen loading rates and the intensity of eutrophication sustained. Contrary to our hypothesis, the results show that, in general, most measures of eelgrass demographics were positively correlated with surface irradiance in all four estuaries. Out of the 45 regression models adjusted between irradiance and demographic variables (density, plastochrone intervals, and above- or below-ground biomass, growth, and production, on both a per shoot and areal basis), only 9 of them were non-significant, and only 6 of those corresponded to the eutrophic estuaries. Most notably, we found a lack of correlation between shoot density and irradiance in the eutrophic estuaries, in contrast to the strong coupling exhibited in estuaries receiving the lowest nitrogen loads. Experimental evidence from previous work has demonstrated severe light limitation and other deleterious impacts imposed by macroalgal canopies on newly recruiting shoots in the eutrophic estuaries, likely contributing to the lack of correlation between shoot density and irradiance at the water's surface. Because the range in eutrophication encompassed by this comparison includes the range of conditions at which eelgrass can survive, the relatively consistent temporal coupling between surface irradiance and most eelgrass demographic variables found here may also be a feature of other shallow temperate systems undergoing increasing eutrophication, and indicates a measure of plant recruitment (density) to be one of the first parameters to become uncoupled from light reaching the water's surface.
  • Preprint
    Macrophyte abundance in Waquoit Bay : effects of land-derived nitrogen loads on seasonal and multi-year biomass patterns
    ( 2008-01) Fox, Sophia E. ; Stieve, Erica ; Valiela, Ivan ; Hauxwell, Jennifer ; McClelland, James W.
    Anthropogenic inputs of nutrients to coastal waters have rapidly restructured coastal ecosystems. To examine the response of macrophyte communities to land-derived nitrogen loading, we measured macrophyte biomass monthly for six years in three estuaries subject to different nitrogen loads owing to different land uses on the watersheds. The set of estuaries sampled had nitrogen loads over the broad range of 12 to 601 kg N ha-1 y-1. Macrophyte biomass increased as nitrogen loads increased, but the response of individual taxa varied. Specifically, biomass of Cladophora vagabunda and Gracilaria tikvahiae increased significantly as nitrogen loads increased. The biomass of other macroalgal taxa tended to decrease with increasing load, and the relative proportion of these taxa to total macrophyte biomass also decreased. The seagrass, Zostera marina, disappeared from the higher loaded estuaries, but remained abundant in the estuary with the lowest load. Seasonal changes in macroalgal standing stock were also affected by nitrogen load, with larger fluctuations in biomass across the year and higher minimum biomass of macroalgae in the higher loaded estuaries. There were no significant changes in macrophyte biomass over the six years of this study, but there was a slight trend of increasing macroalgal biomass in the latter years. Macroalgal biomass was not related to irradiance or temperature, but Z. marina biomass was highest during the summer months when light and temperatures peak. Irradiance might, however, be a secondary limiting factor controlling macroalgal biomass in the higher loaded estuaries by restricting the depth of the macroalgal canopy. The relationship between the bloom-forming macroalgal species, C. vagabunda and G. tikvahiae, and nitrogen loads suggested a strong connection between development on watersheds and macroalgal blooms and loss of seagrasses. The influence of watershed land uses largely overwhelmed seasonal and inter-annual differences in standing stock of macrophytes in these temperate estuaries.