Plant genotype and nitrogen loading influence seagrass productivity, biochemistry, and plant–herbivore interactions

Abstract

Genetic variation within and among key species can have significant ecological consequences at the population, community, and ecosystem levels. In order to understand ecological properties of systems based on habitat-forming clonal plants, it is crucial to clarify which traits vary among plant genotypes and how they influence ecological processes, and to assess their relative contribution to ecosystem functioning in comparison to other factors. Here we used a mesocosm experiment to examine the relative influence of genotypic identity and extreme levels of nitrogen loading on traits that affect ecological processes (at the population, community, and ecosystem levels) for Zostera marina, a widespread marine angiosperm that forms monospecific meadows throughout coastal areas in the Northern Hemisphere. We found effects of both genotype and nitrogen addition on many plant characteristics (e.g., aboveground and belowground biomass), and these were generally strong and similar in magnitude, whereas interactive effects were rare. Genotypes also strongly differed in susceptibility to herbivorous isopods, with isopod preference among genotypes generally matching their performance in terms of growth and survival. Chemical rather than structural differences among genotypes drove these differences in seagrass palatability. Nitrogen addition uniformly decreased plant palatability but did not greatly alter the relative preferences of herbivores among genotypes, indicating that genotype effects are strong. Our results highlight that differences in key traits among genotypes of habitat-forming species can have important consequences for the communities and ecosystems that depend on them and that such effects are not overwhelmed by known environmental stressors.

Grazer Functional Roles, Induced Defenses, and Indirect Interactions: Implications for Eelgrass Restoration in San Francisco Bay

“…While tanks with Ilyanassa, Idotea, and Phyllaplysia all showed an increase in the size of eelgrass shoots during the initial single grazer period, only those containing Idotea were associated with an increase in concentration of phenolic compounds. In addition to consuming epiphytic algae, Idotea consumes some eelgrass, as we observed and as seen in other studies [21,51,52]. This limited herbivory appears to induce a chemical defensive response in the eelgrass in the form of increased production of phenolic compounds.…”

“…These are a structurally diverse, broad category of chemical compounds that may defend against herbivory, as well as reduce biofouling and disease [18,19]. Several studies have demonstrated a negative correlation between concentration of phenolic compounds in seagrass and herbivory on it [20,21]. On marine algae, invertebrates have been found to induce defenses, which can influence interspecific competition among herbivores; e.g., grazing by the gastropod Littorina obtusata induces a defense response in the brown alga Fucus vesiculosus, reducing palatability to several other grazer species [22].…”

Genetic distance predicts trait differentiation at the subpopulation but not the individual level in eelgrass, Zostera marina

“…Leaf phenolic content may be correlated with herbivore feeding preference on eelgrass (Buchsbaum, Valiela & Swain, 1984; Tomas et al., 2011; Vergés, Becerro, Alcoverro & Romero, 2007). We analyzed total phenolic content using approximately 4 mg of dried, ground leaf material from each genotype (pooled from three leaves) following a modified Folin–Ciocalteu method (see Bolser, Hay, Lindquist, Fenical & Wilson, 1998).…”

“…We extracted phenolics with 2 ml of 80% methanol for 24 hr, and then quantified them with a spectrophotometer using caffeic acid as a standard. Ferulic and caffeic acids are two of the most abundant phenolics in Z. marina (Quackenbush, Bunn & Lingren, 1986; Vergeer & Develi, 1997), and previous work showed that caffeic, ferulic, or gallic acids standards for eelgrass phenolic content from shoots collected in Bodega Bay produced similar results (Tomas et al., 2011). …”

“…Multiple genotypes potentially interact at an even finer scale, because as many as four unique genotypes can grow highly intertwined in a 10 cm by 10 cm area (J. Abbott and J. Stachowicz, unpublished data). Previous work shows that eelgrass genotypes differ in traits such as individual growth rate, nutrient uptake, susceptibility to herbivores, and detrital production (Hughes, Stachowicz & Williams, 2009; Tomas et al., 2011), and that these trait differences can predict assemblage performance (Stachowicz et al., 2013). Although much of the differentiation in traits observed in eelgrass across tidal heights is due to phenotypic plasticity (Dennison & Alberte, 1986; Li, Kim, Kim, Kim & Lee, 2013), there can be genetic differentiation in eelgrass growing at different depths (Kim et al., 2017; Ort et al., 2012), suggesting that there may be genetically based trait variation across depths.…”