Minimal impact from long‐term dredge spoil disposal at a dispersive site in Tasman Bay, New Zealand

Abstract

The effects of dredge spoil disposal on contaminant concentrations and benthic macrofauna were examined at a shallow marine disposal site used for c. 20 years. The site had received c. 50 000 m 3 yr -1 of maintenance dredgings annually, from the Port of Nelson. Port sediments were contaminated to varying degrees with some trace metals, organochlorine pesticides, polychlorinated biphenyls, and polycyclic aromatic hydrocarbons. They showed mildly elevated toxicity in laboratory bioassays, and their macrofauna was dominated by small-bodied polychaetes. Despite this, there was very little indication of impact in the spoil disposal area. The disposal area, spoil spreading zone and control sites were all similar in terms of sediment contaminants, sediment toxicity, neogastropod imposex, and macrofauna. The lack of discernible impact is probably because of the dynamic sedimentary environment in the disposal area, which disperses dumped dredgings and mixes them with ambient sediment.

Historical Data Reveal 30-Year Persistence of Benthic Fauna Associations in Heaviy Modified Waterbody

“…Dredging and spoil disposal generally increases turbidity, changes sediment composition and mobilizes toxic materials such as heavy metals (Marmin et al, 2014). The nature of the impact of disposing dredge spoils on the benthic fauna varies with site specific environmental factors such as wave exposure and sediment dynamics (Roberts and Forrest, 1999;Bolam et al, 2010). The management of dredging and disposal of spoils would also be of relevance for new infrastructure projects, including the proposed Tidal Lagoon Swansea Bay, since maintenance dredging may be necessary after operation commences.…”

Cover letter RE “Video observations of the acute effects of dredged sediment deposition on mobile epifauna”

“…Benthic fauna inhabiting these nearshore sites are adapted to disturbances by waves [15, 18], and as Video 2 demonstrates, we observed crabs to maneuver with dexterity in oscillatory flow strong enough for sediment resuspension. Organisms inhabiting sandy substrates also generally recover comparatively faster from disturbance than those at mud or silt substrates [19, 47, 48]. The energetic nearshore zone at the study site is naturally dispersive and so minimizes negative water quality impacts such as heightened turbidity [46, 49].…”

Establishment of a Dredged Material Disposal Site in the Exclusive Economic Zone: New Zealand

“…Biological characteristics: An understanding of the baseline state of the ecosystem is essential to identify significant species as well as in determining post-disposal impacts on Downloaded by [University of Waterloo] at 07:50 04 November 2014 Table 5 Median grain size (µm) and sorting of sediment from the center of the proposed site (3-1), as well as of sediments from 6 samples collected by Bioresearches (2009) (Roberts et al, 1998;Smith & Rule, 2001;Simonini et al, 2005) and also by quantifying changes in contaminant levels of an indicator species (Hirsch et al, 1978;Roberts & Forrest, 1999). Biological assessments that include actual identification of species present on the northeast New Zealand shelf are rare.…”