Rangitoto Channel Dredging: Benthic Ecology Monitoring

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1 Rangitoto Channel Dredging: Benthic Ecology Monitoring 27- Paul Kennedy 1, Emily Jones 2 and Katrina Griffiths 2 1 Golder Associates (NZ) Limited, Auckland, New Zealand; pkennedy@golder.co.nz 2 Golder Associates (NZ) Limited, Auckland, New Zealand Abstract Ports of Auckland Limited were granted resource consent to deepen Rangitoto Channel (the shipping channel approach to Port of Auckland) in 21, representing one of the few large-scale coastal disturbance activities in New Zealand. The dredging works carried out between November and March 27 allowed larger container vessels with drafts of 12.5 m to approach the port during all tides. The consent conditions required Ports of Auckland Limited to monitor the recovery of benthic communities in the channel. Monitoring was carried out using a BACI (Before-After, Control-Impact) design, with baseline surveys carried out in 22 and and post-dredging surveys carried out in 27, 28, 21 and. The monitoring programme investigated sediment texture, and abundance and diversity of invertebrate macrofauna. Results showed increasing similarity of benthic communities between the control and impact sites over time following dredging. Recolonisation progressed to a more stable community with similar characteristics to the predredging communities. Variability in substrate composition and community composition at control sites provided a valuable reference for assessing changes in the dredged channel. Keywords: Ports of Auckland, Rangitoto Channel, dredging, recolonisation, monitoring. 1. Introduction Ports of Auckland Limited undertook dredging of the shipping lane in Rangitoto Channel in 27 to enable larger ships carrying 4,1 TEUs (shipping containers) to approach the port at all tides. The previous all-tide maximum for ships accessing the Channel was 2,9 TEUs. Dredging of Rangitoto Channel occurred from through to 27 to create a shipping lane that was approximately 2 m wide and sufficient for the one-way passage of container ships with a draught up to 12.5 m. As the former shipping lane varied in depth from 11 m to more than 2 m, localised dredging of specific areas was required. Dredging was followed by sweeping of the Channel to smooth the seafloor. The Resource Consent issued by Auckland Council set out a monitoring programme for the assessment of environmental effects of dredging on the seabed in Rangitoto Channel. The monitoring programme comprised six sites for repeated sampling over time. The programme included two baseline surveys prior to dredging, carried out in and, and post-dredging monitoring following completion of dredging and sweeping in 27 with surveys undertaken in 27, 28, 21 and. This paper presents a summary of the results from the monitoring programme and provides a discussion on the biological changes that occurred after dredging. 2. Methods 2.1 Location Six sampling sites were located in Rangitoto Channel, with three site positioned within the area where dredging occurred (i.e., impact sites) and three sites positioned adjacent to the channel in areas that were not dredged (i.e., control sites). These sites were marked by the installation of permanent marker plates on the seafloor which consisted of a 1 m 2 steel plate held in place by 2 m long steel pins inserted into the seafloor. Sites within the channel were labelled as Benthic Impact (BI) 1, 2 and 3, while sites outside the dredged area were labelled as Benthic Control (BC) 1, 2 and Sample collection At each sampling site, a 3 m transect line was set up running from the marker plate at a bearing perpendicular to the direction of the channel. Replicate sets of sediment samples were collected at randomly-assigned distances along this transect by scuba divers. Five replicate sets of samples were collected from each site during the first baseline survey in, while three replicate sets were collected from each site during the second baseline survey in and during the postdredging monitoring surveys in 27, 28, 21 and. Each set of samples consisted of one sample for sediment texture analysis and another for examination of the benthic fauna community. Samples were collected by hand by scuba divers using box cores (22 cm x 22 cm x 14 cm depth). Photographs of the seafloor were also taken at each sampling site, where visibility permitted. The samples for sediment texture analysis were dried to a constant weight at 6 C and the percentage of each size fraction was calculated on dry weight basis. Sediment texture was partitioned into mud (<63 µm), sand (63 µm 2 mm) and gravel (>2 mm) fractions. 1

2 The samples for benthic infauna were each washed through a 5 µm sieve and the remaining contents were preserved and identified to the lowest possible taxonomic level and counted. 2.3 Data analysis The physical characteristics of the seabed substratum were assessed using the proportions of mud, sand and gravel. A one-way Analysis of Variance (ANOVA) was used to test for significant differences between sites. Fauna abundance (i.e., total number of individuals), species richness (i.e., total number of taxa) and species diversity were compared between sites and between years. Species diversity was calculated using the reciprocal of the Berger-Parker Dominance Index. Multivariate statistical procedures were performed on the benthic community dataset using PRIMER 6 statistical software [1]. All multivariate analyses were undertaken using log(x+1) transformed community data and using a Bray Curtis similarity matrix. The multivariate procedures performed included Non-metric Multidimensional Scaling (MDS), Analysis of Similarities (ANOSIM), cluster analysis (CLUSTER) and Species Contributing to Similarity (SIMPER). 3. Results 3.1 Seabed sediments A comparison of the sediment composition at control and impact sites between pre- and postdredging surveys is presented in Figure 1. ANOVA results comparing the sediment dataset between through to is provided in Table 1. There was a high level of variability in the composition of sediment within Rangitoto Channel, illustrated by fluctuations in the relative proportions of sediment types between all years. Table 1 Analysis of variance p-value results from comparison of sediment composition with previous surveys (α =.5). Site Survey Year Gravel (>2 mm) fraction BC BC BC BI BI2..83 No sediment.3 BI Sand (63 µm 2 mm) fraction BC BC BC BI BI No sediment.75 BI Mud (<63 µm) fraction BC BC BC BI BI2..76 No sediment.2 BI At impact sites BI1 and BI3 the proportions of gravel decreased and the sand and mud fractions increased after dredging. There was no soft sediment present at BI2 after dredging. The sediment that returned to BI2 from 21 onwards was significantly different from the composition recorded in from predominantly gravel and sand sediments to seafloor sediments that were mostly mud and sand. Increases in the proportion of sand were recorded at most sites between and, with the exception of BC3. The increase in the sand fraction was exceptionally notable between 21 and, while the proportions of mud and gravel decreased over that time period. BC3 was different from the other sites owing to an increase BC1 BI1 BC2 BI2 BC3 BI3 Figure 1 Sediment composition at control and impact sites in Rangitoto Channel from to. (Source: [3]). Sand composition increased over time at control sites, while mud increased at impact sites after dredging. There was no soft sediment at Site BI2 in 27. 2

3 in the proportion of gravel from to 21, but thereafter the proportion of sand increased and the proportion of gravel had decreased by. 3.2 Benthic fauna Benthic community structure Multi-dimensional scaling (MDS) analysis using averaged site data, shown in Figure 2, indicated that there was a relatively high variability among benthic communities at the sites sampled, with changes in community structure occurring over time at both the impact and control sites. MDS analysis using replicate sample data, shown in Figure 3, indicated that there was also relatively high variability in the benthic assemblages within sites. The MDS analysis showed that even prior to dredging there was some difference between the baseline benthic communities at the impact and control sites. 27 Impact 28 Impact Pre-Impact Control Control Pre-Impact 28 Control Control 27 Control Impact 21 Control 2D Stress:.1 21 Impact Site Control Impact Figure 2 MDS analysis of benthic communities at control and impact sites between and. (Source: [3]). The benthic communities at control and impact sites prior to dredging showed similarity in composition. The recolonised benthic communities were reaching a comparable level of similarity by 21 and. Figure 3 MDS analysis of benthic assemblages in replicate samples collected between and (Source: [3]). The benthic community at the level of individual samples was highly variable as shown by the spread of samples in the plot. The greatest difference between the benthic communities at control and impact sites was recorded in 27 from the first post-dredging survey (Figure 2). The CLUSTER dendogram further illustrated the greater degree of dissimilarity between control and impact sites immediately after dredging (Figure 4). SIMPER analysis determined that similarity between control and impacts sites in 27 was very low at 19.7%, with control sites being more similar to each other between years than with impact sites for the corresponding year. ANOSIM analysis indicated that there was a clear difference but still with some overlap in benthic communities that were present before and after dredging with a Global R-value of Similarity Impact 27 Impact 28 Control 27 Control Pre-Impact Control Pre-Impact Control 21 Impact 21 Control Control Impact Dredging Pre-dredging Post-dredging Figure 4 A dendogram produced by CLUSTER analysis showing the similarity/dissimilarity in benthic community structure between control and impact sites from to (Source: [3]). There was a greater degree of dissimilarity between benthic assemblages at control and impact sites immediately after dredging. The second post-dredging survey in 28 showed more similarity between impact and control sites, which was a trend that carried on through 21 and (Figure 2). SIMPER analysis showed that similarity between control and impact sites proceeded to increase progressively over the survey years (37.1% in 28, 48.% in 21 and 51.7% in ). From 21 onwards, the similarity in benthic communities at impact and control sites increased to close to baseline levels, with up to 51.7% similarity between control and impact sites in. ANOSIM analysis for 21 and datasets indicated that while there were some differences between impact and control sites, there was also some overlap in community structure (Global R-values of.359 and.69 respectively). ANOSIM analysis using sites grouped as baseline or dredged returned a Global R-value of.5 indicating that, overall, while there are some spatial and temporal difference in benthic communities within the channel, these difference were not statistically significant. Further, within site variability generally decreased over time from a high level of variability immediately after 3

4 dredging in 27 to the collection of more similar replicate samples within each site in (Figure 3). However, the structure of the communities have changed notably such that a different composition of benthic species was present at all sites by compared to baseline assemblages, as shown in the separation of sites in the MDS analyses (Figures 2 and 3). ANOSIM analysis confirmed that the changes in benthic communities over time was significant (Global R-value of.73), although with some overlap in community structure between and 28. Temporal changes in fauna abundance at control sites included increases in the abundance of Arthropoda and Annelida and declines in the abundance of other groups including Echinodermata and Mollusca, as shown in Figure 5. The changes in abundance were similar at impact sites with Annelida and Arthropoda increasing by compared to and, while the abundance of other fauna groups, particularly the Mollusca, decreased over time. Taxonomic composition (%) 1% 9% 8% 7% 6% 5% 4% 3% 2% 1% % BC1 BC2 BC3 BI1 BI2 BI3 Site + Year Annelida Arthropod Echinodermata Mollusca Porifera Other Fauna Macroalgae Figure 5 Taxonomic composition at control and impact sites before dredging ( and ) and after recolonization () (Source: [3]). Temporal changes in macrofauna abundance included increases in Arthropods (crustaceans) and Annelids (worms). The changes at impact sites were similar to control sites Fauna abundance, species richness and diversity The abundance of benthic fauna at impact sites declined from to 27, before a notable increase in 28 as shown in Figure 6. In 28 there was very high fauna abundance at impact sites due to the presence of the mussel Arcuatula senhousia, specifically at BI3. This mussel is a non-indigenous species that is known to occur as a dominant benthic species forming dense mats of as many as 5, 1, mussels per m 2 (Crooks 22 [2]). Removing A. senhousia from the dataset indicated that average fauna abundance at impact sites in 28 was consistent with that recorded in 27. Average abundance (+ SE) Year Control Impact Figure 6 Average abundance (number of individuals ± standard error) for control and impact sites during the baselines (, ) and post-dredging monitoring surveys (27-) (Source: [3]). The abundance of benthic fauna at impact sites declined from to 27, before a notable increase in 28. By average fauna abundance at impact sites was similar to that recorded prior to dredging in and. The average fauna abundance at the controls sites showed significant variation across all years. The highest average abundance at the control sites was recorded in 21, with fauna abundance in and 27 also relatively high compared to lower abundances recorded in, 28 and. Average fauna abundance has generally been lower at impact sites than control sites both before and after dredging, with the exception of relatively high fauna abundance in 28. The variation between average fauna abundance at control and impact sites was lowest prior to dredging in and highest after dredging in 27. The variation in abundance between control and impact sites has generally decreased with time since dredging. Species richness has been consistently lower at impact sites than at control sites throughout the study, shown in Figure 7. Average richness (+ SE) Year Control Impact Figure 7 Average species richness (number of species ± standard error) for control and impact sites during the baseline surveys (, ) and the post-dredging monitoring surveys (27-). (Source: [3]). The level of species richness at impacts sites in was within the range of baseline richness values recorded in and. 4

5 Richness declined at impact sites from to 28, then increased in 21 to similar levels recorded in before declining slightly in. The level of species richness at impacts sites in was within the range of baseline richness values recorded in and. There was a small amount of variation in average species diversity between impact and control sites for each year, with the exception of 28, as shown in Figure 8. The average species diversity at impact sites declined from to 28 before increasing in 21 to within the range of baseline levels recorded in and. Average species diversity at control sites gradually increased over time before decreasing in to within the range of baseline levels prior to dredging. The level of diversity recorded in was similar to that recorded in the first baseline survey in. Average species diversity (+ SE) Year Control Impact Figure 8 Average abundance (number of individuals + standard error) for control and impact sites during the baseline surveys (, ) and the post-dredging monitoring surveys (27-). (Source: [3]). The average species diversity at impact sites declined from to 28 before increasing in 21 to within the range of baseline levels recorded in and. 4. Discussion Despite the high natural variability of the benthic communities, the assemblages recorded at the first post-dredging survey undertaken in 27 showed the largest difference between impact and control sites, which could be expected due to the recent removal of the seabed surface in the shipping lane. Continued similarity between control and impact sites in 28, 21 and suggest that benthic communities at impact sites were regenerating to display biological characteristics that were similar to nearby control areas. Benthic community composition across control and impact sites has shown a shift across the monitoring years, both prior to and after dredging of the channel. Some of the species present and the relative abundance of all species in the communities present in are considerably dissimilar to communities present in previous surveys. The composition of the seafloor sediment has also undergone a shift towards a more sand dominated substrate which is a contributing factor to the shift in community composition across control and impact sites. Faunal assemblages recovering from disturbance can go through successional stages of colonisation as benthic communities re-establish themselves. Such successional stages often begin with a sharp increase peak in abundance of a few opportunistic species, followed by a change to the community of lower abundance and increasing diversity, eventually leading to a more stable community structure over time [5]. The length of time over which successional recolonisation occurs is variable; often strongly linked to re-establishment of a similar physical environment to that occurring prior to the disturbance, with significant deviations from the physical characteristics likely to lead to a different community composition. For instance, a study of recolonisation of soft and hard substrates south of Kuwait found that recolonisation of mixed sand/rock habitat was attained in two to five years, while recolonisation of sand substrates took two to six years [3]. The benthic faunal communities of the Rangitoto Channel have likely been moving through expected recolonisation stages that are shifting towards a more stable end community. The shift in both control and impact communities are most likely influenced by changes in the channel bed physical characteristics such as shell and sand movement across the sites. The monitoring has also indicated there is a high degree of natural variability within the benthic communities of the Rangitoto Channel, which is evident both over time and spatially within sites (i.e., highly variable replicate data). Despite this variability the difference in communities between control and impact sites, within a single sample year, have become smaller with time since dredging was undertaken. Overall, the survey data collected in the six to seven years following the last dredging related site disturbance has shown that the benthic community has a composition that is characteristic of the undisturbed community. The length of time over which this recolonisation has occurred is comparable to those seen in a range of international studies. 5. Conclusion The increasing similarity of communities between control and impact sites, and the decrease in variability between sampling replicates indicated that recolonisation in dredged areas has moved on from initial stages of recolonisation and towards a more stable communities with similar characteristic 5

6 (such as level of diversity, richness and abundance) to those of communities prior to dredging. Changes in faunal communities at control sites demonstrate that natural variability of benthic communities and are likely to the result of the changes to substrate sediment composition. Overall, the benthic communities of the Rangitoto Channel are highly variable with influences from both natural variability and the effects of dredging. The seafloor in the dredged area of the channel has been recolonised with a benthic assemblage that is representative of the wider harbour area. 6. References [1] Clarke, K.R. and Gorley, R.N. (26). PRIMER v6: User Manual/Tutorial. PRIMER-E, Plymouth. [2] Crooks, J.A. (22). Predators of the invasive mussel Musculista senhousia (Mollusca: Mytilidae). Pacific Science Vo. 56, No. 1, pp [3] Golder (). Rangitoto Channel Dredging Final Benthic Monitoring Survey. Report prepared by Golder Associates (NZ) Limited for Ports of Auckland Limited. pp. 21. [4] Jones, D.A. and Nithyanandan, M. (). Recruitment of marine biota onto hard and soft artificially created subtidal habitats in Sabah Al-Ahmad Sea City, Kuwait. Marine Pollution Bulletin Vol. 72, pp [5] Lu, L. and Wu, R.S.S. (2). An experimental study on recolonisation and succession of marine macrobenthos in defaunated sediment. Marine Biology Vol. 136, pp