A proposed extension of boundaries at Cape Rodney-Okakari Point Marine Reserve, New Zealand, based on sediment and benthic species distribution
A proposed extension of boundaries at Cape Rodney-Okakari Point Marine Reserve, New Zealand, based on sediment and benthic species distribution
Abstract
This study explores the ecological dynamics of Cape Rodney-Okakari Point Marine Reserve, New Zealand, specifically focusing on sediment composition, depth, and benthic species diversity within and beyond the reserve. Disparities in species diversity across the two locations demonstrate the need for reserve boundary extension from 0.8 km to 3.8 km. Sampling procedures employed a Smith-McIntyre grab across three transects, offering insights into habitat variations. Despite sample collection in 2024, data from 2018 and 2020 was used, revealing insights into the distinct sediment types and species distribution across both locations. Although mollusc-bivalves and mollusc-gastropods were the dominant phyla, with their distribution influenced by depth and sediment characteristics, multiple species showed limited distribution due to these factors. The results support our hypothesis that these limitations result in less biodiversity within the reserve, underscoring the necessity for extending protected areas. Furthermore, the study acknowledges the advantages and disadvantages of grab sampling, emphasising its crucial role in studying benthic communities.
1. Introduction
Cape Rodney-Okakari Point Marine Reserve, New Zealand, hosts vast marine habitats and biodiversity communities. The reserve currently safeguards up to 0.8 km offshore, though we wish to extend its boundaries to 3.8 km. On January 31st 2024, students of Marine 305 from the University of Auckland took part in grab sampling of seafloor sediments in and out of the reserve. However, due to the extensive time required to observe the samples, data from November 2018 and December 2020 were used for analysis.This investigation explores the ecological dynamics within and beyond the reserve, offering opportunities to explore relationships between sediment composition, depth and benthic species diversity. By doing so, we aimed to understand the niches of marine life exclusive to the outer reserve areas, potentially justifying the proposed boundary extension. The motivation behind this extension lies in our hypothesis that the existing boundaries create disparities in species diversity and distribution within and outside the reserve, resulting in limited species protection. Additionally, we aimed to contribute to the local understanding of marine ecology at Cape Rodney and discover future strategies for conservation.2. Methods and Materials
Samples were taken using a Smith-McIntyre grab with a 0.1m^2 sampling area in each location across three transects.In reserve (IR): T1=U9-Y9, 15m-19m, T2=U29-Y29, 26m-30m, and T3=I45-M45, 22m-27m.Out of Reserve (OR): T1=AA9-BE9, 21m-40m, T2=AA29-BA29, 32m-46m, T3=O45-AS45, 30m-50m.This process involved deploying the grab from the University of Auckland's research vessel, Te Kaihōpara. The collected samples were analysed using a 4mm mesh sieve to separate species from the sediment. The sediment type was determined by weight and visual analysis, while collected species were identified and recorded by abundance.3. Results
Eight distinct sediment types were identified within and outside the reserve (Fig. 2a and 2b). Among these, the four coarsest sediment types—stony sand, shell gravel, fine shell gravel, and sand—were exclusively found within the reserve. Excluding stony sand, every sediment type was prevalent beyond the reserve’s boundaries. Muddy sediment exhibited the most widespread distribution in all transects outside the reserve. Exclusive sediment occurrences were observed within specific transects, with sand and fine sand restricted to T1, while shelly fine sand and muddy shell were exclusive to T3-IR. While coarser sediments were common in T3-IR, finer sediments, such as shelly fine sand, muddy shells, and muddy sand were most common in T3-OR.Mollusc-bivalves and mollusc-gastropods were the predominant phyla within and outside the reserve, boasting a higher species count than other phyla, as depicted (Fig. 3a and 3b). In T2-IR and T3-IR, mollusc-bivalves exhibited four species. While T1-IR had only three species, this count increased to five in T1-OR. Similarly, mollusc-gastropods, present across all sampled sites, displayed a notable rise in species counts from T3-IR to T3-OR. Meanwhile, crustacean-true crabs demonstrated a higher species count outside the reserve, particularly in T3-OR. Exclusive findings included bryozoans in T3-IR, while crustacean-mantis shrimp, crustacean shrimp, crustacean-amphipods, crustacean-isopods, and priapulids were exclusively identified outside the reserve. Additionally, a singular species of priapulid was collected at the deepest grab site.
Fig 1. The study site of Cape Rodney-Okakari Point Marine Reserve. Black dashed lines represent the reserve's boundaries, while the red dashed lines represent the proposed extension. Transect 1: U9-BE9, Transect 2: U29 - BA29, Transect 3: I45-AS45.
Fig 2. Sediment types (based on the grab samples taken in November 2018 and December 2020) found across the 3 transects at Leigh, New Zealand. Sediment ranges from muddy sand (finest) to stony sand (coarsest). A = In reserve, B = Out of Reserve.
Fig 3. Distribution of phyla/classes found across the different transects/depths (m) from the coastline during grab sampling in November 2018 and December 2020 at Cape Rodney-Okakari Point. A = In reserve, B = Out of reserve).4. Discussion
4.1. Variation of depth, species and sediment (IR-OR)
The distribution of sediment types across the reserve's transects suggests potentially lower wave energy inshore, leading to coarser sediments (Traykovski et al., 2015). Bryozoans, exclusive to T3-IR, likely resided in this area due to flourishment in low-deposition habitats (Wood et al., 2012). The increasing species count of mollusc-bivalves in T1-OR may be attributed to sediment type. In T1-IR, sand was the only sediment present, while T1-OR consisted of finer sediments. Due to the digging nature of bivalves, a suitable substrate is required, and it is feasible that the finer sediments in T1-OR allowed for a slightly greater diversity of mollusc-bivalve species beyond the reserve (Lutz, 2004). The increase in species counts for mollusc-gastropods from T3-IR to T3-OR could be attributed to the same factor. This sediment variation may allow for a greater range of species to inhabit T3-OR, as mollusc-gastropods have been observed to select habitats based on diet (Brown, 2010). This variability in diet could be due to greater biodiversity beyond the reserve.With crustacean-amphipods being exclusive outside the reserve, this is likely due to finer sediment being preferred amongst the specific amphipod species sampled: Ampelisca chiltoni and Liljeborhia banhami (Taylor & Morrison, 2008). Additionally, due to depth preference, crustacean-mantis shrimp, crustacean shrimp and priapulids were exclusive to the outer reserve. Heterosquilla koning, the species of crustacean-mantis shrimp sampled, prefers a depth between 29-335m alongside sandy substrate, hence the presence in T2-OR at 32.1m (Ahyong, 2012). Similarly, crustacean shrimp have been observed to prefer deeper waters due to decreased predation, allowing a longer lifespan (King & Butler, 1985). Crustacean-true crabs having a higher species count, specifically in T3-OR, are likely attributed to the muddier sediments for burrowing ease, providing refuge from predation (Rice & Chapman, 1971). Similarly, Priapulopsis australis, the species of priapulid found at the deepest sample site, has been previously reported in muddy sand up to depths of 400m (Schmidt-Rhaesa, 2013), likely for burrowing ease and competition avoidance (Van Der Land, 1970). Though only one species of crustacean-isopod (Natatolana rossi) was found at T1-OR, this can be attributed to the abundance of muddy sand beyond the reserve and predatory behaviour. N. rossi has been observed to burrow into soft sediments and wait for available prey before attacking (Marsden, 1999).We could protect more species by extending the boundaries from 0.8km to 3.8km, covering variations in habitats, including depth and sediment types. Protecting areas up to 50m in depth will allow us to conserve the ecology and biodiversity of deeper-dwelling species, including amphipods, shrimp, true crabs and isopods.4.2. Advantages and disadvantages of grab sampling
Grab sampling in marine research is particularly suitable for capturing epibenthos and organisms inhabiting shallow sediments; the process is environmentally friendly, as it targets specific locations while generally leaving specimens unharmed. It is an effective technique for studying epifauna and infauna, providing valuable insights into benthic communities. However, grab sampling comes with limitations. It can be relatively expensive, restricting widespread use and can be hazardous, potentially leading to safety concerns during deployment. The method samples only a small area at a time, and sediment penetration is limited compared to other techniques, struggling with substrates like rubble or cobble. Additionally, the requirement for a boat equipped with a winch can further increase the complexity and cost.Dredging, though able to sample larger areas, raises environmental concerns due to seafloor and habitat destruction. Another alternative is the suction sampler, similar to grab sampling but limited to depths of >20m and requires divers. Despite challenges, the suction sampler is a sustainable option compared to the environmental concerns associated with dredging. Researchers should carefully assess trade-offs and choose the most suitable method based on study requirements and ecological context.4.3. Conclusions
The investigation into sediment types and benthic species distribution at Cape Rodney-Okakari Point Marine Reserve has advanced our understanding of the ecological interactions within this marine environment. The correlation between sediment composition, depth, and species presence underscores the importance of considering these factors in marine conservation efforts. The proposal to extend the reserve boundaries is crucial in safeguarding an extensive array of habitats and ensuring the conservation of diverse species, particularly those residing at greater depths. While our research sheds light on the efficacy of grab sampling in studying benthic communities, we must consider the limitations of grab sampling when interpreting data.
References
Ahyong, S. T. (2012). The marine fauna of New Zealand: Mantis shrimps (Crustacea:Stomatopoda). National Institute of Water and Atmospheric Research (Niwa).Brown, K. M. (2010). Mollusca. In Elsevier eBooks (pp. 277–306). https://doi.org/10.1016/b978-0-12-374855-3.00010-8King, M. G., & Butler, A. (1985). Relationship of life-history patterns to depth in deep-water caridean shrimps (Crustacea: Natantia). Marine Biology, 86(2), 129–138. https://doi.org/10.1007/bf00399018Lutz, R. A. (2004). Bivalve Molluscs: Biology, Ecology and Culture. By Elizabeth Gosling. Fishing News Books. Oxford and Malden (Massachusetts): Blackwell Science. $129.99. x + 443 p; ill.; subject and species indexes. ISBN: 0–85238–234–0. 2003. The Quarterly Review of Biology, 79(3), 317. https://doi.org/10.1086/425799Maas, A. (2012). Nematomorpha, priapulida, kinorhyncha, loricifera. In De Gruyter eBooks. https://doi.org/10.1515/9783110272536Marsden, L. D. (1999). Feeding, respiration, and aerial exposure in a scavenging cirolanid isopod from New Zealand. Journal of Crustacean Biology, 19(3), 459–466. https://doi.org/10.2307/1549254Rice, A., & Chapman, C. J. (1971). Observations on the burrows and burrowing behaviour of two mud-dwelling decapod crustaceans, Nephrops norvegicus and Goneplax rhomboides. Marine Biology, 10(4), 330–342. https://doi.org/10.1007/bf00368093Taylor, R. B., & Morrison, A. (2008). Soft‐sediment habitats and fauna of Omaha Bay, northeastern New Zealand. Journal of the Royal Society of New Zealand, 38(3), 187–214. https://doi.org/10.1080/03014220809510554Traykovski, P., Trowbridge, J., & Kineke, G. C. (2015). Mechanisms of surface wave energy dissipation over a high‐concentration sediment suspension. Journal of Geophysical Research: Oceans, 120(3), 1638–1681. https://doi.org/10.1002/2014jc010245Van Der Land, J. (1970). Systematics, Zoogeography, and Ecology of the Priapulida.Wood, A. C. L., Probert, P. K., Rowden, A. A., & Smith, A. M. (2012). Complex habitat generated by marine bryozoans: a review of its distribution, structure, diversity, threats and conservation. Aquatic Conservation: Marine and Freshwater Ecosystems, 22(4), 547–563. https://doi.org/10.1002/aqc.2236
