Identifying "vital attributes" for assessing disturbance-recovery potential of seafloor communities

Despite a long history of disturbance–recovery research, we still lack a generalizable understanding of the attributes that drive community recovery potential in seafloor ecosystems. Marine soft‐sediment ecosystems encompass a range of heterogeneity from simple low‐diversity habitats with limited biogenic structure, to species‐rich systems with complex biogenic habitat structure. These differences in biological heterogeneity are a product of natural conditions and disturbance regimes. To search for unifying attributes, we explore whether a set of simple traits can characterize community disturbance–recovery potential using seafloor patch‐disturbance experiments conducted in two different soft‐sediment landscapes. The two landscapes represent two ends of a spectrum of landscape biotic heterogeneity in order to consider multi‐scale disturbance–recovery processes. We consider traits at different levels of biological organization, from the biological traits of individual species, to the traits of species at the landscape scale associated with their occurrence across the landscape and their ability to be dominant. We show that in a biotically heterogeneous landscape (Kawau Bay, New Zealand), seafloor community recovery is stochastic, there is high species turnover, and the landscape‐scale traits are good predictors of recovery. In contrast, in a biotically homogeneous landscape (Baltic Sea), the options for recovery are constrained, the recovery pathway is thus more deterministic and the scale of recovery traits important for determining recovery switches to the individual species biological traits within the disturbed patch. Our results imply that these simple, yet sophisticated, traits can be effectively used to characterize community recovery potential and highlight the role of landscapes in providing resilience to patch‐scale disturbances.Peer reviewe

Old Tools, New Ways of Using Them: Harnessing Expert Opinions to Plan for Surprise in Marine Socio-Ecological Systems

Copyright © 2019 Gladstone-Gallagher, Hope, Bulmer, Clark, Stephenson, Mangan, Rullens, Siwicka, Thomas, Pilditch, Savage and Thrush. With globally accelerating rates of environmental disturbance, coastal marine ecosystems are increasingly prone to non-linear regime shifts that result in a loss of ecosystem function and services. A lack of early-detection methods, and an over reliance on limits-based approaches means that these tipping points manifest as surprises. Consequently, marine ecosystems are notoriously difficult to manage, and scientists, managers, and policy makers are paralyzed in a spiral of ecosystem degradation. This paralysis is caused by the inherent need to quantify the risk and uncertainty that surrounds every decision. While progress toward forecasting tipping points is ongoing and important, an interim approach is desperately needed to enable scientists to make recommendations that are credible and defensible in the face of deep uncertainty. We discuss how current tools for developing risk assessments and scenario planning, coupled with expert opinions, can be adapted to bridge gaps in quantitative data, enabling scientists and managers to prepare for many plausible futures. We argue that these tools are currently underutilized in a marine cumulative effects context but offer a way to inform decisions in the interim while predictive models and early warning signals remain imperfect. This approach will require redefining the way we think about managing for ecological surprise to include actions that not only limit drivers of tipping points but increase socio-ecological resilience to yield satisfactory outcomes under multiple possible futures that are inherently uncertain

Biomass-dependent seagrass resilience to sediment eutrophication

Seagrass beds provide a wealth of ecosystem services that benefit society (e.g., habitat and feeding ground for juvenile fisheries species), but the Anthropocene has seen to a global decline of these productive habitats. Many temperate estuaries are becoming eutrophic due to horticultural, agricultural, and urban nutrient run off, but the role of this enrichment in seagrass decline is not fully understood. In a multi-site manipulative field experiment (Tauranga Harbour, New Zealand; 37°S, 176°E), we elevated pore water nitrogen (N) concentrations (mimicking the consequences of long-term eutrophication) to examine effects on seagrass meadows. At six intertidal seagrass sites with differing sediment properties and macrofaunal communities, slow release urea fertiliser (200 g N m−2) was buried in 1 m2 plots at the start of the peak growing season (early summer). After 60 d, we measured several seagrass morphological variables (cover, leaf length and width, and above and below ground biomass), sediment properties, and macrofauna community structure. Results demonstrate that the resilience of seagrass meadows to N enrichment is highly site-dependent. Two of the six sites showed significant declines in a multivariate indicator of seagrass morphology, driven by marked reductions in seagrass cover and leaf length (of up to 78%). Whereas, other sites appeared resilient to N enrichment. It was expected that these site-specific responses would be correlated with changes in sediment properties that alter nutrient processing capacity (permeability and biogeochemistry). However, site-specific responses were instead correlated with the ambient seagrass biomass and macrofaunal diversity. Sites with low ambient seagrass biomass and macrofaunal diversity were less resilient to enrichment. These results highlight that seagrass biomass could be a good indicator of resilience to nutrient enrichment, and the biomass where resilience is lost may lie between 140 and 285 g DW m−2. This study contributes knowledge that is required for predicting and mitigating future impacts of estuarine eutrophication on seagrass ecosystems

Effects of Detrital Subsidies on Soft-Sediment Ecosystem Function Are Transient and Source-Dependent

<div><p>Detrital subsidies from marine macrophytes are prevalent in temperate estuaries, and their role in structuring benthic macrofaunal communities is well documented, but the resulting impact on ecosystem function is not understood. We conducted a field experiment to test the effects of detrital decay on soft-sediment primary production, community metabolism and nutrient regeneration (measures of ecosystem function). Twenty four (2 m²) plots were established on an intertidal sandflat, to which we added 0 or 220 g DW m^-2 of detritus from either mangroves (<i>Avicennia marina)</i>, seagrass <i>(Zostera muelleri)</i>, or kelp (<i>Ecklonia radiata</i>) (n = 6 plots per treatment). Then, after 4, 17 and 46 d we measured ecosystem function, macrofaunal community structure and sediment properties. We hypothesized that (1) detrital decay would stimulate benthic primary production either by supplying nutrients to the benthic macrophytes, or by altering the macrofaunal community; and (2) ecosystem responses would depend on the stage and rate of macrophyte decay (a function of source). <i>Avicennia</i> detritus decayed the slowest with a half-life (t₅₀) of 46 d, while <i>Zostera</i> and <i>Ecklonia</i> had t₅₀ values of 28 and 2.6 d, respectively. However, ecosystem responses were not related to these differences. Instead, we found transient effects (up to 17 d) of <i>Avicennia</i> and <i>Ecklonia</i> detritus on benthic primary production, where initially (4 d) these detrital sources suppressed primary production, but after 17 d, primary production was stimulated in <i>Avicennia</i> plots relative to controls. Other ecosystem function response variables and the macrofaunal community composition were not altered by the addition of detritus, but did vary with time. By sampling ecosystem function temporally, we were able to capture the <i>in situ</i> transient effects of detrital subsidies on important benthic ecosystem functions.</p></div

nMDS ordination of untransformed macrofaunal community data.

<p>Ordinations (based on Bray-Curtis similarity) show species distributions as a function of <b>(A)</b> time: 4, 17 and 46 d post-detrital addition (n = 24) and <b>(B)</b> detrital treatments: control, <i>Avicennia</i>, <i>Zostera</i>, and <i>Ecklonia</i> (n = 18). Each data point represents the macrofaunal community in one core sample.</p

Decay rates of <i>Avicennia</i>, <i>Zostera</i> and <i>Ecklonia</i> detritus.

<p>Data represent the mean percentage (±1 SE; n = 4) of initial dry weight (DW) remaining in litterbags as a function of time.</p

Sediment properties and macrofaunal community variables.

<p>Variables are reported as a function of detritus treatment (control, <i>Avicennia</i>, <i>Zostera</i>, <i>Ecklonia</i>) and time (4, 17, 46 d post-detrital addition). Day 4 ambient data were included to test for procedural effects (see text) and data represent the mean ±1 SE (n = 6 (4 for ambient plots)).</p

Summary of repeated measures PERMANOVA results on univariate measures of ecosystem function.

<p>PERMANOVA tests (Euclidean distance) were performed on ecosystem function variables, as a function of time (4, 17, 46 d post-addition) and treatment (C = control, A = <i>Avicennia</i>, E = <i>Ecklonia</i>, Z = <i>Zostera</i>). Significant effects (<i>p</i> < 0.05) are indicated in bold. In the instance of time × treatment interactions, <i>p</i> values are not given for main effects, and PERMANOVA post-hoc pair-wise tests show treatment effects on each sampling date, separately.</p

Solute fluxes in control and detrital treatments at 4, 17, and 46 d post-addition.

<p><b>(A)</b> NH₄⁺ flux (light and dark chamber fluxes pooled); <b>(B)</b> Net primary production (NPP; white bars light chambers) and sediment oxygen consumption (SOC; black bars dark chambers); and <b>(C)</b> Gross primary production normalised for chlorophyll <i>a</i> biomass (GPP_{chl a}), as a function of treatment (C = Control, <i>A</i> = <i>Avicennia</i>, <i>Z</i> = <i>Zostera</i>, <i>E</i> = <i>Ecklonia</i>) and time (4, 17, and 46 d post-addition). Data represent the mean +1 SE (n = 6). PERMANOVA pair-wise test results (within a sampling date) for significant time × treatment interaction are shown as letters above bars, where bars sharing the same letter are not significantly different (<i>p</i> < 0.05).</p

Light, temperature, and salinity at the sediment-water interface.

<p>For light and temperature, the mean (±1 SE; n = 4 loggers) for each incubation period is presented, and for salinity, the results of a single measurement are shown.</p