Subtidal seagrass surveys at Slipper and Great Mercury Islands

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File Identifier: BC7E74AC-D99E-3580-E055-000000000001 Parent Identifier: BC7DF607-1C8E-32FF-E055-000000000001 Hierarchy Level: feature Hierarchy Level Name: Feature of Subtidal seagrass surveys at Slipper and Great Mercury Islands Abstract: 2019 extent of seagrass meadows at South Bay (Slipper Island) and Huhuri Harbour (Great Mercury Island) estimated by Clark and Crossett (2019). Statement: Subtidal seagrass meadows are rare in New Zealand, with most seagrass now found only in intertidal areas of estuaries. While subtidal seagrass beds may have once been common in subtidal channels of sheltered estuaries, permanently submerged meadows in New Zealand are now primarily restricted to offshore islands (Turner & Schwarz 2006a). The Waikato region has three known areas of subtidal seagrass; Huruhi Harbour (Great Mercury Island), South Bay (Slipper Island) and Whangapoua Harbour. The Huruhi Harbour and South Bay beds were surveyed in the 1970s (Grace & Whitten 1974; Grace & Grace 1976) and in 2004 (Schwarz et al. 2006) and were recently checked by divers in 2017. Waikato Regional Council (WRC) commissioned the Cawthron Institute to carry out a non-destructive field survey of the subtidal seagrass meadows at Huruhi Harbour and South Bay. The aim of the survey was to delineate the extent of the subtidal seagrass meadows and describe the ecological health and condition of the meadows. New Zealand has only one species of seagrass, Zostera muelleri (previously known as Z. capricorni or Z. novaezelandiae; Jacobs et al. 2006). Seagrass meadows are recognised as having high ecological value and are regarded as one of the most valuable coastal ecosystems in terms of the ecological services they provide (Costanza et al. 1997). They are highly productive habitats, supporting the wider coastal area via net export of organic material (Hailes 2006) and accounting for 15% of the net global CO2 uptake by marine biota (Duarte & Chiscano 1999). Seagrass beds also act as a sink for terrestrially-derived nutrients (Short 1987) and stimulate nutrient cycling (Pellikaan & Nienhuis 1988). Their rhizomes and roots stabilise the sediment, while the three-dimensional canopy promotes sediment deposition, contributing to improvements in water quality (Fonseca 1996; Heiss et al. 2000). The structure provided by seagrass meadows, in what is often an otherwise homogenous, soft-sediment environment, also influences the diversity, abundance and spatial distribution of flora and fauna (Henriques 1980; Turner et al. 1999; van Houte-Howes et al. 2004). At Slipper Island, twice as many taxa and more than three times the number of individuals have been found within the seagrass bed compared with adjacent bare sediments (Schwarz et al. 2006). The seagrass bed also provided sleeping grounds for a number of fish species, including adult red mullet and northern bastard red cod (Pseudophycis breviuscula; Schwarz et al. 2006). Seagrass meadows also provide important nursery functions for juvenile fish (Morrison et al. 2014a) and there is evidence that subtidal beds may be more important in this role than intertidal beds (Morrison & Francis 2001). The seagrass beds at Great Mercury Island were found to support high abundances of sand gobies, juvenile yellow-eyed mullet and snapper, with juvenile snapper densities the highest recorded in any habitat in New Zealand (Schwarz et al. 2006). Seagrass meadows have declined in extent worldwide (Short & Wyllie-Echeverria 1996), and New Zealand is no exception (Inglis 2003). Between the 1920s and 1970s, significant declines in seagrass extent took place in estuaries and harbours around Whangarei, Auckland, Whangamata, Tauranga and Christchurch (Inglis 2003). Subtidal seagrass beds have been particularly affected, with 90% of subtidal seagrass lost in Tauranga Harbour (Park 1999), which suggests conditions have become less suitable for the growth of permanently submerged plants (Inglis 2003). Causes of seagrass loss are often attributed to declines in water clarity and quality associated with human activities. In particular, increased sediment and nutrient loads can degrade the light environment through increased water turbidity and the stimulation of phytoplankton, macroalgae and epiphytes (Short & Wyllie-Echeverria 1996). Seagrass beds can also be impacted by the release of toxic compounds in coastal waters (e.g. oil spills, industrial discharge) and direct mechanical damage from activities such as dredging, coastal development and anchoring (Short & WyllieEcheverria 1996). Other factors that may impact seagrass meadows include severe storms, overgrazing and/or competition from natural or introduced species and fungal wasting disease (Matheson et al. 2009). Fungal wasting disease is caused by the marine slime mould Labyrinthula zosterae and is thought to be responsible for the catastrophic die-off of Zostera marina meadows along the Atlantic coasts of North America and Europe during the 1930s (Ralph & Short 2002). Labyrinthula was detected in New Zealand in the 1960s and may have been linked to widespread losses of seagrass in harbours during this period (Armiger 1964). Since then, Labyrinthula has been found in seagrass populations throughout New Zealand (Armiger 1965; Woods & Schiel 1997; Ramage & Schiel 1999; Gillespie et al. 2012a, 2012c, 2012b; Berthelsen et al. 2016; Šunde et al. 2017). Blooms may occur when conditions are favourable (low light, warm temperatures, high salinity; Ralph & Short 2002) and seagrass may be more susceptible when it is stressed due to adverse environmental conditions or anthropogenic impacts (Turner & Schwarz 2006a). Recent research has demonstrated that seagrass is more vulnerable to infection by Labyrinthula when exposed to elevated nitrate concentrations and herbicides, providing support for the hypothesis that disease outbreaks may be linked to increased use and runoff of fertilisers and herbicides (Hughes et al. 2018). Given the vulnerability of seagrass meadows to environmental change, effective management of these habitats requires the collection of accurate information on their distribution and condition (McKenzie et al. 2001; Turner & Schwarz 2006a). This study maps the extent and location of the seagrass meadows at South Bay, Slipper Island and Huruhi Harbour, Great Mercury Island. At each site, we collected information on key parameters that indicate the health and condition of seagrass (seagrass cover, leaf length, above-ground biomass; Duarte & Kirkman 2001) as well as indicators of stress (cover of macroalgae and epiphytes and the severity and prevalence of fungal wasting disease). Results were compared with previous surveys, as well as other seagrass meadows within New Zealand, with the aim of understanding how these subtidal meadows have changed over time and providing a baseline for variables not previously measured. We also trialled the use of visual biomass assessment techniques as non-destructive and rapid method of estimating above-ground biomass. Other Information: Keywords: Biodiversity,Coastal,Ecosystems,Marine,Ocean,Survey Topic Category: biota:flora and/or fauna in natural environment oceans:features and characteristics of salt water bodies (excluding inland waters) Resource Reference Date: 02/03/2021 Metadata Constraints Access Constraints: license Use Constraints: license Maintenance Information Maintenance frequency: notPlanned Last Load Date: 02/03/2021 Spatial Representation Information Recommended Scale: Reference System: EPSG2193 Spatial Representation Type: vector Vector Grid Geographic Bounding Box GeometricObjectType : surface geometricObjectCount : 3 topologyLevel : geometryOnly Number of Dimensions : 2 Cell Geometry : Transformation Param Available : N westBoundLongitude : 1861338.101 eastBoundLongitude : 1847900.8034 southBoundLatitude : 5894481.7206 northBoundLatitude : 5946066.1823 Responsible Parties Role: author Position: Organisation: Cawthron Institute Delivery Point: Private Bag 2 City: Nelson Postal Code: 7042 Country: New Zealand Phone: 64 3 548 2319 Fax: Email: Role: owner Position: Organisation: Waikato Regional Council - Science, Policy and Information Delivery Point: 160 Ward Street City: Hamilton Central Postal Code: 3204 Country: New Zealand Phone: 0800 800 401 Fax: 07 859 0998 Email: inforeq@waikatoregion.govt.nz Data Constraints Data Access Constraints : license Data Use Constraints: license Data Distribution: Unrestricted Licence Type : CC BY 4.0 Licence: https://creativecommons.org/licenses/by/4.0 Licence Expiry: Licence Conditions: Copyright : This work is licensed under a Creative Commons Attribution 4.0 International License © Waikato Regional Council 2019. Subtidal seagrass surveys at Slipper and Great Mercury Islands Data. CC BY 4.0. Data Disclaimers : This technical report has been prepared for the use of Waikato Regional Council as a reference document and as such does not constitute Council’s policy. Council requests that if excerpts or inferences are drawn from this document for further use by individuals or organisations, due care should be taken to ensure that the appropriate context has been preserved, and is accurately reflected and referenced in any subsequent spoken or written communication. While Waikato Regional Council has exercised all reasonable skill and care in controlling the contents of this report, Council accepts no liability in contract, tort or otherwise, for any loss, damage, injury or expense (whether direct, indirect or consequential) arising out of the provision of this information or its use by you or any other party. Data Use Limits : Download Links URL Format Notes Supporting Information Links URL Format Notes  https://waikatoregion.govt.nz/services/publications/tr201929/ PDF Subtidal seagrass surveys at Slipper and Great Mercury Islands TR 2019/29 General Notes: Attribute Information Name Description Notes Restricted Primary Key OBJECTID N Y LOCATION Location of survey N N TECHNICAL_REPORT Link to published technical report on the Waikato Regional Council website N N GEOMETRY N N