Subtidal Forests Research Articles

Nutrient conditions determine the strength of herbivore-mediated stabilizing feedbacks in barrens.

Abiotic environmental conditions can significantly influence the way species interact. In particular, plant-herbivore interactions can be substantially dependent on temperature and nutrients. The overall product of these relationships is critical for the fate and stability of vegetated ecosystems like marine forests. The last few decades have seen a rapid spread of barrens on temperate rocky reefs mainly as a result of overgrazing. The ecological feedbacks that characterize the barren state involve a different set of interactions than those occurring in vegetated habitats. Reversing these trends requires a proper understanding of the novel feedbacks and the conditions under which they operate. Here, we explored the role of a secondary herbivore in reinforcing the stability of barrens formed by sea urchin overgrazing under different nutrient conditions. Combining comparative and experimental studies in two Mediterranean regions characterized by contrasting nutrient conditions, we assessed: (i) if the creation of barren areas enhances limpet abundance, (ii) the size-specific grazing impact by limpets, and (iii) the ability of limpets alone to maintain barrens. Our results show that urchin overgrazing enhanced limpet abundance. The effects of limpet grazing varied with nutrient conditions, being up to five times more intense under oligotrophic conditions. Limpets were able to maintain barrens in the absence of sea urchins only under low-nutrient conditions, enhancing the stability of the depauperate state. Overall, our study suggests a greater vulnerability of subtidal forests in oligotrophic regions of the Mediterranean and demonstrates the importance of environment conditions in regulating feedbacks mediated by plant-herbivore interactions.

Improved estimates of net primary production, growth, and standing crop of Macrocystis pyrifera in Southern California.

The giant kelp Macrocystis pyrifera forms subtidal forests on shallow reefs in temperate regions of the world. It is one of the fastest-growing multicellular autotrophs on Earth and its high productivity supports diverse marine food webs. In 2008, we published a method for estimating biomass and net primary production (NPP) of giant kelp along with five years of data, to provide a more integrated measure of NPP than those yielded by previous methods. Our method combines monthly field measurements of standing crop and loss rates with a model of kelp biomass dynamics to estimate instantaneous mass-specific growth rates and NPP for each season of each year. We have since improved our approach to account for several previously unresolved sources of biomass loss. These improvements have led to a near doubling of our prior estimates of growth and NPP. At our site with the most persistent stand of giant kelp, NPP averages ~5.2kgdrymass·m-2 ·yr-1 and results from the rapid growth (~3.5% per d) of a relatively small standing biomass (~0.4kgdry mass/m2 on average) that turns over ~12times annually. Here we provide revised estimates of seasonal biomass, growth, and NPP for the five years covered by our previous publication (2002-2006), along with more than a decade of additional data (2007-2017). We also present updated relationships for predicting giant kelp biomass and NPP from much more easily obtained measurements of frond density. These data can be used to understand the mechanisms that drive variation in giant kelp NPP at a wide range of temporal scales. No copyright or proprietary restrictions are associated with the use of this data set other than citation of this Data Paper.

Concomitance of oligotrophy and low grazing pressure is essential for the resilience of Mediterranean subtidal forests

The study aimed at investigating factors influencing the recovery of the canopy seaweed Cystoseira brachycarpa. A manipulative experiment was done to test if in barren patches the recovery of Cystoseira I) is enhanced by the removal of the urchins, II) is prevented by eutrophication, III) depends on the time of patch clearance and IV) decreases with the distance from Cystoseira bed edge within the barren patch. The effects of the same factors on the structure of the macroalgal assemblage were also tested. Cystoseira recovered abundantly only in clearings where nutrients were not added and urchins were removed. Furthermore, Cystoseira recovered irrespectively of the time the patches were cleared and the distance from the canopy edge. This study showed that the lack of sea urchins at oligotrophic conditions was essential for Cystoseira brachycarpa recruitment, providing evidence that interacting constraints are involved in the recovery of Cystoseira beds.

Variability in kelp forest structure along a latitudinal gradient in ocean temperature

Subtidal forests comprised of kelps and other canopy-forming macroalgae represent critically important marine habitats. Kelp forests exhibit high rates of primary productivity, magnified secondary productivity, support high levels of biodiversity and provide various ecosystem services. Compared with many other regions, kelp forests around the UK have been largely understudied despite their recognised importance and the possible impacts of environmental change factors. We conducted surveys at 12 kelp-dominated open-coast sites within four regions in the UK, spanning ~9° in latitude and ~3°C in mean sea temperature. We used a combination of quadrat-scale abundance and percent cover data as well as transect-scale canopy cover estimates to quantify ecological structure at multiple spatial scales. Kelp forest structure varied significantly between sites (nested within region) and also between regions. Regional-scale differences were principally driven by a higher abundance/cover of Alaria esculenta at the colder northern regions (i.e. north and west Scotland), and the presence of the Lusitanian kelp Laminaria ochroleuca at some sites in the southernmost region (i.e. southwest England) but nowhere else. The kelp Laminaria hyperborea dominated all sites and varied significantly between sites but not regions. All assemblage-level and population-level response variables were highly variable between sites within regions, suggesting that environmental factors varying at corresponding spatial scales (e.g. wave exposure, turbidity, sedimentation) are important drivers of pattern. The detection of regional-scale variability suggests that predicted changes in ocean climate, particularly increased sea temperature, may lead to changes in kelp forest structure in the future, with poleward range contractions (for A. esculenta) and expansions (for L. ochroleuca) likely. However, as the distribution-abundance patterns of the assemblage dominant L. hyperborea did not vary predictably with ocean temperature at this spatial scale, the fundamental structure of these habitats may be more influenced by localised factors, at least in the short-term. The relative importance of multiple, concurrent environmental change factors in structuring UK kelp forests remains largely unknown.

Loss and Recovery Potential of Marine Habitats: An Experimental Study of Factors Maintaining Resilience in Subtidal Algal Forests at the Adriatic Sea

BackgroundPredicting and abating the loss of natural habitats present a huge challenge in science, conservation and management. Algal forests are globally threatened by loss and severe recruitment failure, but our understanding of resilience in these systems and its potential disruption by anthropogenic factors lags well behind other habitats. We tested hypotheses regarding triggers for decline and recovery potential in subtidal forests of canopy-forming algae of the genus Cystoseira.Methodology/Principal FindingsBy using a combination of historical data, and quantitative in situ observations of natural recruitment patterns we suggest that recent declines of forests along the coasts of the north Adriatic Sea were triggered by increasing cumulative impacts of natural- and human-induced habitat instability along with several extreme storm events. Clearing and transplantation experiments subsequently demonstrated that at such advanced stages of ecosystem degradation, increased substratum stability would be essential but not sufficient to reverse the loss, and that for recovery to occur removal of the new dominant space occupiers (i.e., opportunistic species including turf algae and mussels) would be required. Lack of surrounding adult canopies did not seem to impair the potential for assisted recovery, suggesting that in these systems recovery could be actively enhanced even following severe depletions.Conclusions/SignificanceWe demonstrate that sudden habitat loss can be facilitated by long term changes in the biotic and abiotic conditions in the system, that erode the ability of natural ecosystems to absorb and recover from multiple stressors of natural and human origin. Moreover, we demonstrate that the mere restoration of environmental conditions preceding a loss, if possible, may be insufficient for ecosystem restoration, and is scarcely cost-effective. We conclude that the loss of complex marine habitats in human-dominated landscapes could be mitigated with appropriate consideration and management of incremental habitat changes and of attributes facilitating system recovery.

Recovering subtidal forests in human‐dominated landscapes

Summary 1. Policy initiatives that seek to recover lost habitats require the capacity to anticipate and suppress the mechanisms that drive loss. The replacement of forested landscapes by simple landscapes comprising of opportunistic or ‘weedy’ species represents an increasingly common phenomenon across human‐dominated systems. The failure of subtidal forests to recover from natural and human disturbance and their ultimate replacement by degraded habitats is recognized globally. The current lack of knowledge on whether such shifts can be reversed jeopardizes considerations of restoration policy within increasingly human‐dominated landscapes. 2. We critically assessed the model that recovery of canopies within remnant kelp forests in degraded landscapes (i.e. turf‐forming algae that carpet space) is slower than in adjacent forested landscapes, but may be increased by removing turfs. 3. After generating experimental disturbance, canopies recovered to their former state within forested landscapes, but not in remnant forests in degraded landscapes. Removal of turfs from spaces between remnant forests, however, enabled canopies to recruit and subsequently develop covers that matched those in remnant forests. 4. Whilst the supply of canopy‐forming propagules to degraded landscapes is likely to decline with gap expansion, we show that improvements to forest resilience and restoration are possible via policies that result in a reduction of turf covers. These results also support the model that regime‐shifts need not be a product of synchronized loss, but can occur as a result of reduced rates of canopy‐recruitment over broad areas and many years. Indeed, patterns of canopy‐loss over several decades redouble attention to the human‐mediated conditions that enable turfs to retain space (i.e. elevated nutrient and sediment loads via coastal runoff). 5. Synthesis and applications. We demonstrate that future restoration is a possible outcome of polices that promote ecosystem recovery. In doing so, we reduce uncertainty about policy initiatives that aim to upgrade the recycling potential of wastewater treatment plants (e.g. nearly 45% of South Australia’s metropolitan wastewater) to improve the quality of water needed to restore subtidal forests. Uncertainty about resilience‐building and restoration management are redressed by demonstrating that the feedbacks maintaining regime‐shifted landscapes are not necessarily permanent.

Variations in the configuration of algae in subtidal forests: Implications for invertebrate assemblages

Abstract The ecology of Australia's most extensive canopy‐forming alga, Ecklonia radiata, is often studied with little regard as to whether it occurs in monospecific stands or as part of a mixed assemblage of canopy‐forming algae. We tested the hypothesis that E. radiata does not primarily occur as monospecific stands, rather it occurs more often in stands of mixed algae. At a 1‐m2 scale we recognized three main configurations within forests of algae (hereafter called stands): E. radiata that occurs as (i) monospecific stands; (ii) clumps (four or more individuals together) surrounded by species of Fucales; or (iii) individual plants (or clusters of fewer than three plants) interspersed among species of Fucales. All three types of stand occurred in similar proportions (percentage cover) across two regions of Australia's southern coastline (Western and South Australia). We also tested the hypothesis that these three types of stands (identified at 1 m2) contain different assemblages of invertebrates associated with the holdfast of E. radiata. Assemblages of invertebrates varied between monospecific and interspersed stands, but not between monospecific and clumped stands. These results suggest that variation in the configuration of subtidal algae (stands measured at a 1‐m2 scale) has the potential to influence the composition and abundance of associated biota. We suggest that although studies in stands of monospecific E. radiata may provide useful information for the majority of forests containing E. radiata (monospecific and clumped stands made up 65% of forests sampled), caution must be used when extrapolating to stands of mixed, interspersed algae (>31% of forests sampled).

Lessonia tholiformissp. nov. (Phaeophyta: Laminariales) from the Chatham Islands, New Zealand

Abstract A new species of Lessonia Bory, L. tholiformis C.H. Hay is described from the Chatham Islands of New Zealand. Unlike most other Lessonia species, the holdfast of L. tholiformis is a semi-solid dome or mound formed by overlapping layers of flattened, lobed haptera. In this respect, L. tholiformis is somewhat similar to L. nigrescens, a species growing in the intertidal region of Chile, but it differs from that species in other ways. Juveniles of L. tholiformis have rugose blades. However, the rugosity wanes with age and blades of mature plants are consistently smooth. L. tholiformis is the only species of Lessonia found growing on the Chatham Islands where it is apparently endemic. In many localities it is the dominant kelp forming thick, subtidal forests to a depth of 10 m. L. tholiformis is the fifth species of Lessonia now recorded from the south-west Pacific region — the other species being: L. variegata J. Ag. (New Zealand mainland), L. brevifolia J. Ag. (Auckland, Campbell, Antipodes, and Bo...