Introduction
Marine ecosystem engineers, such as seagrass, coral, kelp, mangroves, and reef-forming shellfish, create biogenic habitat structure that increase biodiversity whilst also providing many other valuable ecosystem services. For example, the habitat provided by shellfish reefs can act as important juvenile fish nurseries (zu Ermgassen et al.
2016; Knoche et al.
2020), stabilise benthic sediments (Commito et al.
2005), reduce water turbidity (Newell
2004), and facilitate nutrient cycling (Ray and Fulweiler
2021; Ray et al.
2021). It is estimated that globally oyster reefs have declined by 85% from their historical extent (Beck et al.
2011), while mussel reefs experienced an overall decline of 53% in Europe, North America, and Australia (Lotze et al.
2006). The widespread decline in these biogenic shellfish populations is likely due to cumulative issues, including overharvesting (Beck et al.
2011), disease (Smith
1985; Beck et al.
2009; Gillies et al.
2018), pollution (Newell
2004), and habitat destruction (Beck et al.
2011). The decline in biogenic shellfish habitat also results in losses of the associated ecosystem services, including the loss of biodiversity, but there can also be wider ecosystem impacts, including trophic disruptions (zu Ermgassen et al.
2006; McLeod et al.
2013; Christianen et al.
2017).
Active human intervention in the form of restoration is increasingly being applied to reinstate biogenic shellfish habitats and with them the ecosystem services they once provided (Coen et al.
2007; zu Ermgassen et al.
2020). However, many studies of the restoration of biogenic shellfish habitats focus on the logistics of restoring the shellfish species without also assessing the ecosystem services provided by the restored shellfish (e.g., Alder et al.
2021; Wilcox et al.
2018; Schotanus et al.
2020; Walles et al.
2016). One key ecosystem service that motivates shellfish restoration efforts is the corresponding increase in biodiversity associated with the restored habitats. However, there is limited understanding of the changes in biodiversity that occurs in these restored habitats (Toone et al.
2021). Scientists have studied biodiversity in wild shellfish habitats and found it to be high compared to adjacent sediment (Commito et al.
2008; McLeod et al.
2013,
2019a). However, biodiversity can depend on many factors, including structural habitat complexity arising from the physical arrangement of shellfish (Grabowski et al.
2005; van der Ouderaa et al.
2021), and aspects of the environment in which the shellfish habitat is located, such as the composition of the underlying benthic sediment (Gray
2002; Commito et al.
2008). For example, the abundance and diversity of both infauna and epifauna in shellfish habitats can be influenced by the grain size profile of the underlying sediment (Commito et al.
2008).
The biodiversity associated with shellfish can be categorized into three distinct ‘faunal classifications’ based on the primary location of occupation within the habitat: infauna, epifauna, and pelagic fauna. Infauna include all organisms living on or in the sediment underlying the shellfish habitat. Epifauna include all organisms, including macroalgae, living on the sediment or on the shellfish, whilst pelagic fauna include all the mobile organisms in the water column above the habitat. These classifications were chosen as they are typically used throughout literature to describe different levels of biodiversity (Willis and Babcock,
2000; Commito et al.,
2008; McLeod et al.,
2013; Sea et al.,
2022). However, there is a tendency for biodiversity studies of shellfish habitats to focus on individual faunal classifications, such as infauna in isolation (e.g., McLeod et al.
2019a,
b), pelagic fauna in isolation (e.g., Willis & Babcock
2000), or a combination of epifauna and infauna (e.g., McLeod et al.
2013; Commito et al.
2008). While these studies improve our understanding of the biodiversity found on wild reefs, examining all three faunal classifications is needed to develop a more complete understanding of the overall changes in biodiversity associated with restoration of shellfish habitats. By understanding biodiversity on restored shellfish habitats, especially at different trophic levels, restoration managers can make informed decisions that can lead to higher ecosystem biodiversity. Additionally, where spatial differences in biodiversity responses can be ascertained, they can be used to greatly improve location selection and overall outcomes from shellfish habitat restoration. This includes targeting important fisheries, maximizing habitat generation, and improving juvenile fish nurseries. In this regard, tracking the response of biodiversity at all three faunal classifications in restored small-scale mussel habitats that are spread across a sediment gradient can be a valuable first step for guiding larger scale restoration efforts.
In New Zealand, the endemic, green-lipped mussel,
Perna canaliculus, is culturally, economically, and environmentally important (Jeffs et al.
1999). This species forms extensive high-density aggregations, or mussel reefs, on both hard and soft benthic substrata in shallow coastal waters in many parts of the country that used to be up to hundreds of square kilometres in extent (McLeod et al.
2012; Paul
2012). Many of the largest mussel reefs in various parts of New Zealand were fished to functional extinction last century, but the species now forms the basis of a major aquaculture industry (FAO
2021). With a ready supply of aquaculture grown mussels, there is growing community and industry interest in the restoration of the mussel reefs lost through overfishing, and a desire to recover some of the lost ecosystem services from the mussel reefs, including biodiversity. A potential restoration location is Pelorus Sound, an extensive drowned river valley system (~ 60 km long) which has undergone further ecological changes since the mussels were removed, including increased sediment loads, lower macroalgal abundances, and decreased fish populations (Handley
2015; Urlich and Handley
2020).
To evaluate the effect of shellfish restoration in Pelorus Sound on infaunal, epifaunal, and pelagic faunal biodiversity a series of small-scale experimental restored mussel plots were placed at four locations spatially distributed over ~ 30 km along Pelorus Sound and subsequent changes in biodiversity of the three faunal classifications were measured over a 1-year period. The four locations included habitats covering a benthic sediment gradient from fine mud to sand and rock. We hypothesised that: (1) the biodiversity observed in the four locations would differ based on the underlying sediment composition (i.e., regardless of mussel addition), and (2) the addition of mussels would alter the benthic environment, increase benthic structural complexity and food availability, and therefore change the structure and abundance of the infaunal, epifaunal, and pelagic faunal communities.
Discussion
The decline in the extent of habitat-forming species, such as reef-forming shellfish, has contributed to a dramatic decline in biodiversity in coastal and marine environments throughout the world (zu Ermgassen et al.
2020). Restoration of these habitat-forming species can help to reinstate the ecosystem services they provide, including increasing the biodiversity and corresponding resilience of coastal ecosystems to future anthropogenic stressors (McLeod et al.
2019b; zu Ermgassen et al.
2020). Pilot-scale restoration has the potential to provide valuable information regarding the effect of shellfish restoration on biodiversity to guide subsequent decisions for larger scale restoration efforts. In this restoration trial, the addition of mussels to the seafloor overall resulted in a general reduction of infaunal species abundance and biodiversity, with a concomitant increase in epifaunal and pelagic species abundances, specifically from those species that benefit from benthic habitat complexity and an increase in food availability.
The markedly lower diversity and abundance of infaunal organisms associated with restoring mussels observed in this current study appear to be a result of differences in organic enrichment and sediment grain size. Increases in organic content and fine benthic sediments have both been shown to alter infaunal biodiversity for a range of habitats (Gray
2002; Kemp et al.
2005; Sciberras et al.
2017; Drylie et al.
2020), although some studies of remnant wild shellfish reefs have found a high abundance and diversity of infauna regardless of the underlying sediment (Commito et al.
2008; van der Ouderaa et al.
2021). The underlying soft sediment at the four experimental locations varied in the quantity of the fine sediment (i.e., clay and silt). Observed changes in organic enrichment and grain size occurred within the first 5-months but there was no difference between 5-and 13-months, which indicates that these changes in the sediment occurred within the first 5-months after installing the mussels and did not continue to accumulate from 5 to 13-months. Mussels produce faeces and pseudo-faeces causing organic enrichment of the surrounding sediment (Commito et al.
2008; Norling and Kautsky
2008; Donadi et al.
2014) and remove suspended fine sediment as they filter water (Christianen et al.
2017). The extent of organic enrichment of sediment in wild shellfish habitats has been shown to vary between different intertidal benthic environments (van der Ouderaa et al.
2021), and in this current study the greatest magnitude change was evident at Grant Bay, which may be due to the coarser composition of the sediment that characterised this site. The sediment organic content recorded in the four study locations was generally higher than measured for wild intertidal mussel habitats in the Netherlands (5.8% mussel, 0.9% control), however when compared to control sites the mussel habitats had a similar, higher amount of silt (1.3 times higher) and sediment organic content (6.4 times higher) as seen in our study (Christianen et al.
2017).While the magnitude of the change in benthic sediment composition associated with the restoration of the mussel habitat varied by location, an overall increase in organic content and fine sediment was always apparent, the timing and extent of which were concordant with an observed general decline in the abundance and diversity of infauna at most locations.
Infaunal organisms respond to changes in sediment grain size and organic enrichment in a variety of circumstances. For example, high density longline aquaculture of mussels can greatly enrich the organic content of the sediment below the farm, typically resulting in a decrease in biodiversity, but an increased abundance in opportunistic deposit feeding organisms, such as members of the deposit feeding polychaete family Capitellidae (Keeley et al.
2009; Keeley
2013). Furthermore, organic enrichment of the sediment lowers the available oxygen and increases sulphide levels, which can create unfavourable conditions for some infauna, such as suspension feeders (Thrush et al.
2003; Drylie et al.
2020; Handley et al.
2020). Sensitive infauna respond quickly and dramatically to a small change in organic enrichment. For example, sediment in a New Zealand estuary was experimentally enriched from 2.1 to 3.7% resulting in an 80% decrease in the abundance of infauna after 70 days (Drylie et al.
2020). This sensitivity in infauna can result in changes in functional groups of infaunal organisms (Greenfield et al.
2016; Drylie et al.
2020) and was demonstrated by polychaetes in this study by a ten-fold higher abundance of free-living scavenger polychaete family Lumbrineridae in the control plots. Whereas suspension and deposit feeding polychaetes of the family Spionidae appeared to be less sensitive, decreasing one-fold under restored enrichment. Deposit feeding capitellids contributed to the highest difference between restored and control plots, being in almost a two-fold higher abundance on the control plots. The overall lower abundance, diversity, and deposit feeding infauna in restored mussel plots in this study is opposite to what has been previously recorded on wild shellfish reefs (Commito et al.
2008; McLeod et al.
2013; van der Ouderaa et al.
2021). However, this study indicates a similar outcome for infauna as reported in these previous studies, that mussels facilitate certain infauna and inhibit others. This inhibition appears to be within the first 5-months for sensitive infauna, while the facilitation that leads to a high abundance of infauna seen in wild reefs may take more than 13-months on restored mussel habitats.
For epifauna, the addition of mussels to the seabed caused an increase in taxonomic richness and abundance across nearly every location within the first 5-months, likely as a result of the increase in habitat complexity, especially the provision of hard surfaces (i.e., mussel shells) provided by the restored mussels themselves. This habitat complexity is associated with elevated abundance and diversity of epibenthic organisms (e.g., Grabowski et al.
2005; McLeod et al.
2013) through the provision of habitat, food, and protection (Coen et al.
2007; Christianen et al.
2017). Despite differences in pre-existing epifauna among study locations, the installation of mussels resulted in the establishment of a similar assemblage of epifauna at all locations by 5-months, and thereafter. This was particularly evident for mobile epifauna, such as sea cucumbers, cat’s eye snails, eleven-armed and cushion seastars, that were all found in higher abundances in the mussel plots, indicating a preference for this restored habitat and the increased food availability it provides. For example, sea cucumbers were likely attracted to mussel plots by increases in organic matter (Slater et al.
2011; Zamora and Jeffs
2011; Sea et al.
2022) and eleven-arm seastars are known mussel predators (Wilcox and Jeffs
2019). These organisms appeared to have migrated into the mussel habitats judging from their large observed sizes. Observed differences among study locations in mobile epifaunal diversity could be the result of differences in nearby source populations. For example, the higher concentration of eleven-arm seastars at Grant Bay may be due its proximity to a marine farm, as higher concentrations of these seastars occur under farms (Inglis and Gust
2003). To aid in the restored mussel survival the seastars were collected and relocated throughout the sampling period, which may have influenced the overall epifaunal numbers. However, the observed continued migration of seastar predators into these small-scale trial plots indicate that it is important to consider location selection for restoration because proximity of sources of mussel predators may impact restoration success (Wilcox and Jeffs
2019).
The Te Mara location had lower abundance, richness, diversity of epifaunal organisms and the lowest macroalgae abundance of all locations. This may be a result of the environmental conditions being unsuitable for macroalgae, given the low macroalgal abundance in general at Te Mara. Shellfish facilitate macroalgal growth due to the enrichment of the benthic environment and the availability of hard substrate to settle and attach (Kemp et al.
2005; Norling and Kautsky
2008). The establishment of macroalgae could be an important process because it has been shown to facilitate mussel recruitment through providing larval and early juvenile settlement surfaces (Buchanan and Babcock
1997; Alfaro et al.
2004). Macroalgae also increase biodiversity by providing habitat and food for other organisms and have been shown to decrease the impact of ocean acidification on marine bivalves (Young and Gobler
2018). In this study a two-fold increase in macroalgal cover was found on the restored plots, which was driven by a large difference in macroalgae abundance between the control and mussel plots by 13-months. This indicates that macroalgae may take longer to establish on restored mussel habitats than other epifauna groups or seasonal factors were more important. The absence of juvenile sessile organisms seen in this study suggests that the establishment of biodiversity through less mobile epifauna may take longer than 13-months.
The installation of mussels substantially increased the abundance of demersal pelagic fauna at the two sampled locations, most likely as a result of attraction to the structure and associated biomass generated by the restored reefs (Powers et al.
2003; Parsons et al.
2016). For example, blue cod and triplefin fish are both demersal fish species that were observed more frequently on the restored mussel plots, although blue cod were only observed at Maori Bay potentially due to limitations in the range of this species (Beentjes et al.
2012). Blue cod are an endemic commercially and recreationally important species in New Zealand’s South Island that are reported to have a significantly depleted population in the study area (Beentjes et al.
2012) and choosing restoration locations within their species ranges is vital for creating habitats for this vulnerable species. Triplefin fish are known to inhabit shellfish aquaculture farms (Morrisey et al.
2006) and are attracted to locations with more structural complexity where they can find refuge and protection (Feary and Clements
2006). These small fish are important trophically as they contribute to the diets of many larger species, including New Zealand’s commercially important snapper (Jones
2013). Therefore, triplefin fish contribute to further higher trophic level pelagic diversity, which is important for ecosystem restoration and long-term resilience. In this study triplefin fish were observed 66 times more frequently on the mussel habitats than on adjacent soft sediment. Similarly, higher numbers of small fishes have been recorded in two studies in the North Island of New Zealand, one on remanent mussel reefs (14 × higher; McLeod et al.
2013) and one on 6-year-old restored mussel habitats (16 × higher; Sea et al.
2022). Blue cod diet consists of crustaceans, molluscs and polychaetes (Jiang and Carbines
2002) and triplefin fish diets consist of small mobile benthic invertebrates like crustaceans and molluscs (Feary et al.
2009), with both known to be opportunistic feeders based on their habitats (Jiang and Carbines
2002; Feary et al.
2009). Shellfish reefs have been shown to alter predator- prey interactions (Donadi et al.
2015) and be important feeding grounds for fish (Lenihan et al.
2001), so the increase of these species on the restored plots may have reduced the resident invertebrate communities. Snapper and eagle rays are known to prey on mussels (Alder et al.
2021), however there was low numbers of snapper recorded overall in this study, which was unlikely to largely impact mussel survival.
Shellfish reefs are also known to serve as nurseries for fish (Knoche et al.
2020; zu Ermgassen et al.
2020), although only adult fish were recorded in this study. The lack of conspicuous fish recruitment in mussel plots may be the result of the small size of the restored plots, the cryptic nature of juvenile fish, or because the ecosystem is not established yet. The differences in pelagic community changes seen between 5-months and 13-months most likely reflect seasonal variation, where fish migrate to Pelorus Sound during the warmer summer temperatures (i.e., 13-month sampling). Not all pelagic fauna respond to restored shellfish habitats in the same way, and this was seen in this study particularly from non-demersal fishes such as spotty wrasse, jack mackerel, and kahawai that were observed in higher abundances in the control plots. Choosing restoration locations within species ranges that will benefit from an increase in benthic habitat complexity may therefore increase the pelagic diversity associated with restored shellfish habitats.
Trial pilot-scale restoration plots are recommended by shellfish restoration global guidelines to assess habitat suitability and understand which restoration techniques are needed before scaling up efforts (Fitzsimons et al.
2020). The results of this study indicate that pilot-scale restoration is also effective in determining biodiversity outcomes that can provide valuable information to guide subsequent decisions for larger scale restoration efforts. However, with the nature of pilot studies, this study was short-term (13-months) and had small sized plots, so the results do not show biodiversity changes over multiple seasons, which may be why there was no identified larval recruitment into the mussel plots. In oyster restoration it has been shown that after 6 years a restored oyster reef matches natural reefs biomass and ecosystem function (Smith et al.
2022), indicating that this an early look into restored mussel habitats and may not be representative of the biodiversity the occurs on restored mussel habitats long-term. Additionally, with the size of the mussel plots, the results are representative of an area of a restored mussel habitat but may not necessarily be representative of a larger scale restored mussel bed. This study does, however, provide the first understanding of what occurs on small-scale restored mussel habitats within the first-year post-restoration at multiple faunal classifications and the results can be used to improve location selection to enhance biodiversity on large scale restoration efforts.
The loss of ecosystem engineers, such as reef building shellfish, is generally recognized as a major threat to marine biodiversity, yet the lack of understanding of the recovery of biodiversity on restored shellfish reefs could limit restoration initiatives. Understanding marine biodiversity outcomes associated with restoring mussel habitats is important for restoring lost ecosystem services, including juvenile fish nurseries and habitat generation, along with long-term ecosystem resilience. From a management perspective, the results provided in this study highlight location-specific factors that should be considered to maximise biodiversity when restoring mussel habitats. Important factors include sediment composition and environmental conditions for macroalgae growth. Potential factors that were observed in this study that could also be considered when choosing locations to restore mussel habitats are nearby sources of species including predators, and ranges of important demersal fish. Small-scale pilot studies, like this one, can be used to inform management and improve overall biodiversity outcomes for future larger scale shellfish restoration. This deeper understanding of the biodiversity response in restored shellfish habitats is invaluable for justifying shellfish restoration initiatives and advancing efforts to restore vital foundation species and the habitats they create.
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