How kelp in drag lose their ruffles: environmental cues, growth kinematics, and mechanical constraints govern curvature

Primary production of the kelp Lessonia corrugata varies with season and water motion: Implications for coastal carbon cycling

Kelp forests provide vital ecosystem services such as carbon storage and cycling, and understanding primary production dynamics regarding seasonal and spatial variations is essential. We conducted surveys at three sites in southeast Tasmania, Australia, that had different levels of water motion, across four seasons to determine seasonal primary production and carbon storage as living biomass for kelp beds of Lessonia corrugata (Order Laminariales). We quantified blade growth, erosion rates, and the variation in population density and estimated both the net biomass accumulation (NBA) per square meter and the carbon standing stock. We observed a significant difference in blade growth and erosion rates between seasons and sites. Spring had the highest growth rate (0.02 g C · blade-1 · d-1 ) and NBA (1.62 g C · m-2 · d-1 ), while summer had the highest blade erosion (0.01 g C · blade-1 · d-1 ), with a negative NBA (-1.18 g C · m-2 · d-1 ). Sites exhibiting lower blade erosion rates demonstrated notably greater NBA than sites with elevated erosion rates. The sites with the highest water motion had the slowest erosion rates. Moreover, the most wave-exposed site had the densest populations, resulting in the highest NBA and a greater standing stock. Our results reveal a strong seasonal and water motion influence on carbon dynamics in L. corrugata populations. This knowledge is important for understanding the dynamics of the carbon cycle in coastal regions.

A Life Outside

How do the morphologies of organisms affect their physical interactions with the environment and other organisms? My research in marine systems couples field studies of the physical habitats, life history strategies, and ecological interactions of organisms with laboratory analyses of their biomechanics. Here, I review how we pursued answers to three questions about marine organisms: (a) how benthic organisms withstand and utilize the water moving around them, (b) how the interaction between swimming and turbulent ambient water flow affects where small organisms go, and (c) how hairy appendages catch food and odors. I also discuss the importance of different types of mentors, the roadblocks for women in science when I started my career, the challenges and delights of interdisciplinary research, and my quest to understand how I see the world as a dyslexic.

Ecological biomechanics of damage to macroalgae

Macroalgae provide food and habitat to a diversity of organisms in marine systems, so structural damage and breakage of thallus tissue can have important ecological consequences for the composition and dynamics of marine communities. Common sources of macroalgal damage include breakage by hydrodynamic forces imposed by ambient water currents and waves, tissue consumption by herbivores, and injuries due to epibionts. Many macroalgal species have biomechanical designs that minimize damage by these sources, such as flexibly reconfiguring into streamlined shapes in flow, having either strong or extensible tissues that are tough, and having chemical and morphological defenses against herbivores and epibionts. If damage occurs, some macroalgae have tissue properties that prevent cracks from propagating or that facilitate tissue breakage in certain places, allowing the remainder of the thallus to survive. In contrast to these mechanisms of damage control, some macroalgae use breakage to aid dispersal, while others simply complete their reproduction prior to seasonally-predictable periods of damage (e.g., storm seasons). Once damage occurs, macroalgae have a variety of biomechanical responses, including increasing tissue strength, thickening support structures, or altering thallus shape. Thus, macroalgae have myriad biomechanical strategies for preventing, controlling, and responding to structural damage that can occur throughout their lives.

Ecological biomechanics of marine macrophytes

Macroalgae and seagrasses in coastal habitats are exposed to turbulent water currents and waves that deform them and can rip them off the substratum, but that also transport essential water-borne substances to them and disperse their propagules and wastes. Field studies of the physical environment, ecological interactions, and life history strategies of marine macrophytes reveal which aspects of their biomechanical performance are important to their success in different types of natural habitats and enable us to design ecologically relevant laboratory experiments to study biomechanical function. Morphology and tissue mechanical properties determine the hydrodynamic forces on macrophytes and their fate when exposed to those forces, but different mechanical designs can perform well in the same biophysical habitat. There is a trade-off between maximizing photosynthesis and minimizing breakage, and some macrophytes change their morphology in response to environmental cues. Water flow in marine habitats varies on a wide range of temporal and spatial scales, so diverse flow microhabitats can occur at the same site. Likewise, the size, shape, and tissue material properties of macrophytes change as they grow and age, so it is important to understand the different physical challenges met by macrophytes throughout their lives.

Flow, form, and force: methods and frameworks for field studies of macroalgal biomechanics

Macroalgae are ecologically important organisms that often inhabit locations with physically challenging water motion. The biomechanical traits that permit their survival in these conditions have been of interest to biologists and engineers alike, but logistical and technical challenges of conducting investigations in macroalgal habitats have often prevented optimal study of these traits. Here, we review field methods for quantifying three major components of macroalgal biomechanics in moving water: fluid flow, macroalgal form, and hydrodynamic force. The implementation of some methodologies is limited due to the current state and accessibility of technology, but many of these limitations can be remedied by custom-built devices, borrowing techniques from other systems, or shifting lab-based approaches to the field. We also describe several frameworks for integrating flow, form, and force data that can facilitate comparisons of macroalgal biomechanics in field settings with predictions from theory and lab-based experiments, or comparisons between flow conditions, habitats, and species. These methods and frameworks, when used on scales that are relevant to the examined processes, can reveal mechanistic information about the functional traits that permit macroalgae to withstand physically challenging water motion in their habitats, using the actual fluid flows, macroalgal forms, and physical forces that occur in nature.

Scaling and Structural Properties of Juvenile Bull Kelp (Nereocystis luetkeana)

Bull kelp (Nereocystis luetkeana), the only canopy-forming kelp in the Salish Sea, provides primary production in the nearshore subtidal environment and serves as an important habitat for economically and ecologically important species. An annual species, each year juvenile bull kelp sporophytes must grow from the hydrodynamically more benign benthos to the water column, where they experience substantial drag at the surface. Because of the differences in morphology and ecology across life stages, and the fact that previous work has focused mainly on adult bull kelp, we tested whether morphology and structural properties change with stipe length, investigating scaling of both juvenile (stipe length < 40 cm) and mature (stipe length > 40 cm) kelp, and testing how juvenile stipes fail. Juvenile bull kelp grow proportionally (isometric growth) when young, but lengthen more quickly than would be predicted by bulb size (negative allometry) at maturity. Based on our data, the predicted breakpoint between isometric and allometric growth occurred at about 33 cm, likely approximately one to two weeks of growth. Cross-sectional area of the stipe, force to failure, work to failure, and stiffness (Young's modulus) all grow more slowly than would be predicted based on length, while maximum stress and toughness increase more quickly than predicted. There is no change in extensibility over the size range we tested, suggesting that this material property does not change with stipe length. The differences in biomechanics between juvenile and adult kelp are likely a response to the varied hydrodynamic environments experienced during the annual life cycle, which highlights the importance of studying organisms across life stages.