Spartina Expt 2013 Manuscript #1


Rachael E. Blake, Jill A. Olin

Abstract

     This will say something brilliant about this experiment. 

Introduction

      This will be the introduction.  I need to come up with an outline for the introduction. 

Something about oil spills.
Something about the history of oil spills and oiling in Louisiana marshes.
Something about the threats to marshes.
Something about the importance of marshes. 

This should be re-ordered.

Methods

                 Experimental Design

                To examine the impacts of multiple chemical and herbivore stressors on the important saltmarsh foundation species, Spartina alterniflora (hereafter Spartina), we conducted a factorial mesocosm experiment in a greenhouse at Louisiana State University (LSU), Baton Rouge, LA, during May - July 2013.  Each mesocosm consisted of one 19 L plastic bucket with a drain hole (plugged with a rubber stopper) near the bottom, inside which a 13.66 cm diameter, 30 cm deep flower pot rested on a brick (to improve drainage).  Clusters of 7 - 8 mature stems and associated root material of Spartina (Vermillion variety) were planted in each flower pot in a substrate comprised of 2.85 L potting soil and 0.95 L sieved estuarine mud collected from the Louisiana Universities Marine Consortium (LUMCON) lab in Cocodrie, LA .  All mesocosms were enclosed by a supported mesh bag cover (0.4mm mesh) fine enough to contain the herbivores.  Treatments receiving oil were segregated in a different section of the greenhouse from treatments not receiving oil to minimize the effects of airborne compounds on the non-oiled mesocosms.

               We conducted the experiment at 20 ppt salinity because this is the approximate salinity of saltmarshes near Grand Isle, LA that have been monitored by the Coastal Water Consortium (cwc.lumcon.edu) since the Macondo oil spill in 2010.  However, because the Spartina had been growing at 0 ppt salinity on the LSU campus in Baton Rouge, we had to acclimate our plants to salt water (made with Instant Ocean©) prior to beginning the experiment by increasing salinity by 5 ppt each week until we reached our target salinity of 20 ppt.  Following acclimation to 20 ppt, we watered the plants every two weeks with 20 ppt water.  This allowed the water levels to drop over the two weeks, simulating a low tide of small amplitude typical of saltmarshes in this geographic region.  When we filled the mesocosms with water at the two-week interval, this simulated a flood tide event where the surface of the marsh was covered with water.  Water loss from mesocosms was through plant transpiration, as the salinity in the mesocosms remained 20 ppt and did not increase as would be expected with evaporation. 

               Chemical stressors included oil (MC252 surrogate crude oil) and chemical disperant (Corexit 9500) applied in three treatments: each chemical individually, and the two chemicals together.  110ml of surrogate crude oil were added to each oil treatment, and 2.2 ml of Corexit 9500 were added to each dispersant treatment.  For treatments receiving both oil and dispersant, the two chemicals were mixed together before being applied to the mesocosms.  Chemicals were painted on the bottom half of the standing aboveground biomass to simulate oiling conditions in coastal saltmarshes during the spill (Mendelssohn 2012), and because other studies have shown impacts when half the stem biomass was oiled (Lin 2012).   To simulate the same action in no-chemical treatments, salt water (20 ppt) was painted on the bottom half of the Spartina.  Any chemicals or salt water remaining after painting were poured on the surface of the sediment in the mesocosm.  The oil dose for each experimental unit was based on the average high oil concentration found in saltmarsh sediments sampled in 2010 in Barataria Bay, Louisiana during the spill (Turner 2014).  We used a dispersant to oil ratio of 1 part dispersant for every 50 parts oil.   This ratio is in the mid-range of published ratios at which dispersant was likely applied during the spill (Kujawinski 2011).   

               Herbivores used in this experiment included the snail Lirroraria irrorata (hereafter Littoraria), and two species of phloem-feeding insects from the genus Prokelisia (Prokelisia marginata and Prokelisia dolus).  The snails are a common and abundant saltmarsh invertebrate in Louisiana, and are strong herbivores that can decrease Spartina biomass by farming fungus on the plant tissue (