Leclerc, J., P. Riera, L. Lévêque, and D. Davoult. 2016. Contrasting temporal variation in habitat complexity and species abundance distributions in four kelp forest strata. Hydrobiologia 777:33-54. DOI:10.1007/s10750-016-2742-6
Kelp, your cozy refuge
You are diving in the kelp forests off the coast of northern France, where the waves are strong, and you are descending amid leafy seaweeds. You see glimpses of bright red and orange sea stars, some sideways-swimming shrimp, and even some scuttling crabs! Or, if you’re me, you may witness these underwater forests in nature documentaries from your comfy lounge-chair.
Kelp provides both a tasty morsel for sea otters and several microhabitats, which are small-scale places for animals to live. Believe it or not, kelp forests represent one of the most complex habitats in coastal waters. Habitat complexity within kelp forests protects organisms from environmental stresses, including wave energy and light intensity. As English muffins have plenty of crannies to hold butter, kelp forests have lots of tiny crevices and spaces. These spaces provide refuges and help trap tasty food particles for snails and worms.
The present study, which took place off the northern coast of France, focused on the kelp Laminaria hyperborea. Considered a habitat of its own, this kelp consists of 4 distinct strata. These include the rocky bottom, the holdfast, which anchors the kelp body to the rock, the stipe, which is the main part of the kelp, and the frond-like lamina (see Fig. 1 below).
What the researchers wanted to know
The researchers wanted to know how much biodiversity existed within the kelp Laminaria hyperborea. They also wanted to know if seasonal changes in seaweed and invertebrate species composition occurred among the four kelp strata.
How did the researchers answer their question?
What better way to sample some kelp than to go SCUBA diving! Divers collected 5 adult L. hyperborea individuals from the coast of Roscoff, France. Divers obtained 5 adult kelp plants each season over the course of one year.
What the researchers found
The researchers found 100 seaweed species and 387 species of invertebrates within the four kelp strata. Seaweeds and mobile invertebrates underwent significant seasonal changes among the four kelp tiers.
Laminae, the highest part of the kelp, which are exposed to the most wave action, held the lowest number of seaweed species. In contrast, stipes (the main body of the kelp) contained the highest seaweed biomass. Stipes also had the most stable seaweed community composition throughout the entire sampling period. This means the researchers encountered many of the same seaweeds from one season to the next. Holdfasts contained high abundances of sessile sponges and ascidians (affectionately called sea squirts). Grazing worms and the predaceous hairy crab also dominated holdfasts. The rock substrate boasted the highest number of mobile invertebrates and seaweeds.
Besides differences in biodiversity among kelp strata, major changes in habitat complexity occurred in the sub-canopy strata (holdfast and rock). Biomass of bushy and rough-leaf seaweeds increased from November to June. During this time, abundance of grazing snails and particle-feeding amphipods (side-swimming shrimp) also increased. The authors suggest that the seaweeds provided more surface area, which directly benefited these mobile animals.
Annual dying off of lamina and attached seaweeds likely influenced invertebrates by creating bridges among microhabitats. The researchers discovered a striking predominance of mobile invertebrates across seasons, which could point to the role of attached seaweeds in connecting different kelp strata and individuals.
Seasonal shedding of the tops of kelp plants and seaweeds created different scenarios for particle-feeders and grazers. Grazing snails, unlike particle-feeding amphipods, prefer fresh seaweed over decaying leaves. The autumn decay of several red algae seaweeds coincided with the decline of specialized grazing snails. In contrast, particle-feeder numbers stayed constant throughout the sampling period. The researchers attributed stable abundance of particle feeders to the continuous supply of small, nutrient-rich particles from dead kelp and other sources of organic (carbon-based) matter.
Why this study is important
This study demonstrated that kelp provides habitats for hundreds of invertebrates and seaweeds, making these seaweeds hotspots of biodiversity. Furthermore, the different kelp strata play an important role in maintaining such high levels of biodiversity. But many of us may not get to see kelp forests up close, so why does this matter?
Kelp forests and their associated communities of seaweeds and invertebrates provide a dynamic interplay of connected microhabitats. Even sessile invertebrates, such as sponges, can serve as habitat for seaweeds or tiny invertebrates. Young developmental stages of invertebrates and fish often seek refuge within kelp forests because the complex habitats provide refuge from predators and physical disturbance.
The bigger picture
One of the lingering questions in my field of aquatic ecology is why we can find so many species coexisting in one place. This study found over 200 invertebrate species living on the kelp, the number of species varying seasonally among the kelp strata. This is a prime example of niche partitioning, where species that compete for similar resources can find their own habitat space and survive. Microhabitats within the kelp strata help retain nutritious organic matter from decaying seaweeds and animals that would otherwise be washed away with the waves. Imagine an all-you-can eat buffet: if your favorite entrée is all gone, you can move down the line to your second, third, fourth choices, etc. Like an endless buffet, kelp forests retain nutrient particles so that a continuous supply of food is available to its diverse tenants, and these organisms can coexist throughout the different microhabitats.
Understanding how habitat complexity drives animal and seaweed communities is important if we wish to protect dynamic habitats such as kelp forests from human disturbance. So, next time you have an English muffin for breakfast, remember, we can all get a little help from kelp.
Kate received her Ph.D. in Aquatic Ecology from the University of Notre Dame and she holds a Masters in Environmental Science & Biology from SUNY Brockport. She currently teaches at a small college in Indiana and is starting out her neophyte research career in aquatic community monitoring. Outside of lab and fieldwork, she enjoys running and kickboxing.