First quantification of subtidal community structure at Tristan da Cunha Islands in the remote South Atlantic: from kelp forests to the deep sea

<div><p>Tristan da Cunha Islands, an archipelago of four rocky volcanic islands situated in the South Atlantic Ocean and part of the United Kingdom Overseas Territories (UKOTs), present a rare example of a relatively unimpacted temperate marine ecosystem. We conducted the first quantitative surveys of nearshore kelp forests, offshore pelagic waters and deep sea habitats. Kelp forests had very low biodiversity and species richness, but high biomass and abundance of those species present. Spatial variation in assemblage structure for both nearshore fish and invertebrates/algae was greatest between the three northern islands and the southern island of Gough, where sea temperatures were on average 3-4^o colder. Despite a lobster fishery that provides the bulk of the income to the Tristan islands, lobster abundance and biomass are comparable to or greater than many Marine Protected Areas in other parts of the world. Pelagic camera surveys documented a rich biodiversity offshore, including large numbers of juvenile blue sharks, <i>Prionace glauca</i>. Species richness and abundance in the deep sea is positively related to hard rocky substrate and biogenic habitats such as sea pens, crinoids, whip corals, and gorgonians were present at 40% of the deep camera deployments. We observed distinct differences in the deep fish community above and below ~750 m depth. Concurrent oceanographic sampling showed a discontinuity in temperature and salinity at this depth. While currently healthy, Tristan’s marine ecosystem is not without potential threats: shipping traffic leading to wrecks and species introductions, pressure to increase fishing effort beyond sustainable levels and the impacts of climate change all could potentially increase in the coming years. The United Kingdom has committed to protection of marine environments across the UKOTs, including Tristan da Cunha and these results can be used to inform future management decisions as well as provide a baseline against which future monitoring can be based.</p></div

First quantification of subtidal community structure at Tristan da Cunha Islands in the remote South Atlantic: from kelp forests to the deep sea

<div><p>Tristan da Cunha Islands, an archipelago of four rocky volcanic islands situated in the South Atlantic Ocean and part of the United Kingdom Overseas Territories (UKOTs), present a rare example of a relatively unimpacted temperate marine ecosystem. We conducted the first quantitative surveys of nearshore kelp forests, offshore pelagic waters and deep sea habitats. Kelp forests had very low biodiversity and species richness, but high biomass and abundance of those species present. Spatial variation in assemblage structure for both nearshore fish and invertebrates/algae was greatest between the three northern islands and the southern island of Gough, where sea temperatures were on average 3-4^o colder. Despite a lobster fishery that provides the bulk of the income to the Tristan islands, lobster abundance and biomass are comparable to or greater than many Marine Protected Areas in other parts of the world. Pelagic camera surveys documented a rich biodiversity offshore, including large numbers of juvenile blue sharks, <i>Prionace glauca</i>. Species richness and abundance in the deep sea is positively related to hard rocky substrate and biogenic habitats such as sea pens, crinoids, whip corals, and gorgonians were present at 40% of the deep camera deployments. We observed distinct differences in the deep fish community above and below ~750 m depth. Concurrent oceanographic sampling showed a discontinuity in temperature and salinity at this depth. While currently healthy, Tristan’s marine ecosystem is not without potential threats: shipping traffic leading to wrecks and species introductions, pressure to increase fishing effort beyond sustainable levels and the impacts of climate change all could potentially increase in the coming years. The United Kingdom has committed to protection of marine environments across the UKOTs, including Tristan da Cunha and these results can be used to inform future management decisions as well as provide a baseline against which future monitoring can be based.</p></div

Linear mixed models testing the effects of island, site, and depth zone on patterns of invertebrate and macroalgal density and lobster biomass among the Tristan da Cunha islands.

<p>Density and biomass were square root transformed. Statistically significant p-values are in bold text. Density and biomass were square root transformed. Presented for the random effect of site are Wald p-values and percent of total variance explained.</p

Map of the study region.

<p>Map depicts the locations of the Tristan da Cunha Islands in the South Atlantic (top) and locations of scuba surveys (<i>n</i> = 34), pelagic camera surveys (<i>n</i> = 26), and deep sea benthic camera surveys (<i>n</i> = 23) in the four islands of the Tristan da Cunha Islands group (bottom).</p

Fish biomass and density-site means.

<p>Bubble plots depicting site-level variation in fish biomass, density, and species composition of kelp forest fish communities in the four Tristan da Cunha Islands. Shown are plots of (A) fish biomass and (B) fish density. Bubble size scales with the biomass or density estimated using site-level means of SCUBA surveys.</p

Results of a PERMANOVA testing differences in fish community structure between depth categories (0–750 m vs. >750 m depth) from the deep-sea drop-cam surveys at the Tristan da Cunha Islands.

<p>Results of a PERMANOVA testing differences in fish community structure between depth categories (0–750 m vs. >750 m depth) from the deep-sea drop-cam surveys at the Tristan da Cunha Islands.</p

Results of a PERMANOVA testing differences in pelagic community structure among islands from the mid-water BRUVS surveys at the Tristan da Cunha Islands.

<p>Data are species abundances (MaxN) and were square root transformed.</p

Invertebrate and algal biomass and density-island means.

<p>Bar plots depicting island-level variation in lobster biomass and the density of common invertebrates and macroalgae in the Tristan da Cunha Islands from SCUBA surveys of nearshore habitats. Shown are mean values for each species ± 1 standard error of the mean.</p

Linear mixed models testing the effects of island (fixed), site (random), and depth zone (fixed) on patterns of fish biomass among the Tristan da Cunha islands.

<p>Density and biomass were square root transformed. Statistically significant p-values are in bold text. Presented for the random effect of site are Wald p-values and percent of total variance explained.</p

Multivariate description of pelagic assemblages in the Tristan da Cunha islands from mid-water BRUVS surveys.

<p>Plots depict canonical analysis of principal coordinates (CAP) analyses based on a Bray-Curtis resemblance matrix site-level species specific abundances. Vectors overlaying the plot depict the species that are driving separation among sites and islands in species composition. Data were square root transformed prior to analysis.</p