Consumption of terrestrial organic matter in the rocky intertidal zone along the central Oregon coast

Abstract

AbstractTerrestrial organic matter (TOM) constitutes an important source of energy in many aquatic environments. This is the first study to compare the consumption of terrestrial leaves (Alnus rubra) to high‐quality (Nereocystis luetkeana) and low‐quality (Fucus gardneri) sources of drift kelp and low‐quality seagrass (Phyllospadix spp.) in pools of the rocky intertidal zone on Cape Arago, Oregon, USA. On average, 1.02 kg ash‐free dry mass (AFDM) per day of TOM was transported by small forested streams during fall leaf abscission to the rocky intertidal zone in small pocket bays. Leaves of A. rubra (red alder) varied in wrack from negligible (12 g AFDM/m2) on some beaches to the primary source of detritus on others (256 g AFDM/m2). Long‐term feeding experiments (months) showed that consumption rates of A. rubra in the spring (19.7% of the initial pack biomass) and in the fall (52% of the initial pack biomass) were intermediate between those of N. luetkeana (fully consumed) and Phyllospadix spp. (never >10% consumed of the initial pack biomass). Long‐term experiments also showed that conditioned fall‐shed leaves of A. rubra were consumed at a faster rate (52.3% of initial pack biomass) than spring‐shed leaves of A. rubra (16.8% of initial pack biomass). Short‐term feeding experiments again showed that consumption of A. rubra in the spring (18.6% of initial pack biomass over 3 d) and fall (16.7% over 7 d) was intermediate between N. luetkeana (57.3% consumed over 3 d in the spring and 60.7% over 7 d in the fall) and Phyllospadix spp. (<1% consumed in the spring). Except for Phyllospadix spp., consumption rates followed polyphenolic concentrations in that, N. luetkeana (418 mg/mL) > A. rubra (3415 mg/mL) > F. gardneri (8098 mg/mL). Phyllospadix spp. was rarely consumed despite a fairly low concentration of phenolics (800 mg/mL). In the temperate zone, leaves of A. rubra may constitute an important source of energy during the fall and winter when more nutritious marine resources dieback, such as single‐celled algae and kelp (e.g., N. luetkeana).