Interactive roles of mesograzers and current flow in survival of kelps

Abstract

ABSTRACT:The relationship between hydrodynamic energy and biological processes is examined for a kelp-dominated marine community in the San Juan Archipelago. Populations of a common and widely distributed kelp, Nereocystis luetkeana, were established and rates and causes of mortality followed at 7 sites differing greatly in tidally driven current velocities and wave exposure (measured by permanently deployed instrument packages). Mortality of N. luetkeana was not related to storm energy, but exhibited a significant non-linear relationship with tidal current energy such that mortality rates were highest at sites exhibiting protracted periods of calm punctuated by episodes of strong currents. The role of mesograzers (primarily the gastropod Lacuna vincta) on survivorship in these kelp populations was evaluated in the field and in laboratory flume experiments. The relationship between grazer damage and stipe breaking force was investigated by measuring the tensile forces required to break experimentally damaged stipes. Although undamaged stipes can easily withstand the tensile forces imposed by even the strongest current and wave exposures, a very small amount of damage will have highly significant negative effects on breaking strength. While apparently responsible for a significant portion of N. luetkeana mortality in low and variable energy environments, L. vincta is unable to persist on kelp stipes in high-energy environments and its role there is trivial. The relationship between L. vincta grazing and hydrodynamic energy is however nonlinear, because water movement has opposite effects on grazer foraging behavior and the drag forces imposed on kelps, and this results in a complex relationship between hydrodynamic energy and kelp survival. We suggest a conceptual model for relating kelp survival to grazing intensity in hydrodynamically variable environments. The model leads to the prediction, exhibited in our field results, that the probability of plant mortality may be maximized in regions of intermediate flow energy. KEY WORDS: Kelp · Nereocystis · Hydrodynamics · Population dynamics · LacunaResale or republication not permitted without written consent of the publisher

Rapid changes in the epifaunal community after detachment of buoyant benthic macroalgae

“…Many seaweed species (mainly Phaeophyceae) possess pneumatocysts that keep submerged algal fronds in an upright position and allow them to float after detachment. When detached from their substratum by wave action (Seymour et al, 1989), herbivorous feeding activity (Duggins et al, 2001), or as a natural event in the alga's life cycle (Thomsen et al, 2004) buoyant macroalgae rise up to the sea surface and carry with them a diverse epifaunal community from the benthic habitat.…”

Restoration of the bull kelp Nereocystis luetkeana in nearshore rocky habitats

“…A H/W ratio between 4.4 and 6.4 is ideal for sites with moderate current velocity (Gardner 1980). Although not measured in this study, Duggins et al (2001) found average current speed at Cantilever Point (-8 m MLLW) to be less than 10 cm s -1 and velocities at Saddlebag Island were suspected to be similar (authors' pers. obs.).…”

“…N. luetkeana forms a natural persistent bed in the north cove of Saddlebag Island (48°32.93' N, 122°33.41' W) in Padilla Bay. At a second site, N. luetkeana previously was established experimentally (Duggins et al 2001) at Cantilever Point on San Juan Island (48°32.84' N, 123°0.23' W) in the San Juan Archipelago. Both sites are characterized by serpentine bedrock that is covered in places by shale and silt.…”

“…Extra care was taken to avoid any damage to the stipe or holdfast, as even a small nick could cause future stipe or holdfast failure (Duggins et al 2001). The rope sections were each threaded through a hard plastic clip and transplants were placed into a seawater-soaked pillowcase and transported to Cantilever Point, across the San Juan Channel.…”

“…Sterilized seawater (125 ml) was added and the suspension was blended for 45 s in 5 to 10 s pulses. This method is commonly used to induce fertility in kelp gametophytes (Vadas 1972, Bolton & Lüning 1982, Tom Dieck 1992, Duggins et al 2001. Sample drops of the blended seed stock were examined microscopically to confirm densities of at least two 3 to 5 cell fragments per 0.15 mm 2 .…”