Spring 2015

Document Type

Master's Thesis

Degree Name

Master of Science (M.S.)


Science & Environmental Policy


Kelp wrack occurs year round along temperate coastlines providing food to a low productivity environment; few efforts however, have been made to quantify wrack’s variability in deposition, persistence, and utilization along beaches and the rocky intertidal. Knowing the biomass deposited, persistence (amount of time a piece of wrack is found in an area), and how primary and secondary consumers respond to wrack allows for a better understanding of how these three variables work together to promote a food web and the ecological implications of the presence/absence of wrack within an area. My work focused on spatiotemporal variation in wrack deposition, persistence, and utilization by kelp flies and bacteria in central California from April 2013 to August 2014. Changes in wrack biomass density, persistence on the shore, phlorotannin concentration, bacterial density, and kelp fly density were measured along with a suite of other factors including species composition, condition (individual or fragmented), and burial. Over the study period, a significant difference in biomass density was found between substrates with higher amounts of wrack on the rocky intertidal. In addition, persistence maps indicating hotspots of prolonged wrack accumulation showed a significant increase in kelp wrack at greater distances from the shoreline, such as the berm on beaches and the high intertidal in the rocky intertidal, although no difference between substrate was observed. In terms of utilization, phlorotannin concentration did not appear to affect bacterial abundance or fly density, which suggested that the levels present in kelp wrack were not a strong deterrent for bacteria and flies. A two-week experiment was also conducted, which manipulated the weight of the giant kelp Macrocystis pyrifera to determine its effect on the above-mentioned factors. The rate of water loss and persistence was measured daily, while utilization changes occurred on a slower scale and were measured weekly. During this time, kelp fly density changed significantly between substrate, while phlorotannins decreased significantly over time. Again, there was no relationship between phlorotannin concentration, bacterial abundance, or fly density, although bacterial abundance and fly density increased when wrack was rehydrated by high tides after initial desiccation. Quantifying deposition, persistence, and utilization of kelp wrack is important to establish baselines, make comparisons, and understand the interactions taking place that can affect wrack’s ecological role as a subsidy.


Thesis (M.S.) Division of Science and Environmental Policy