INTERIM
REPORT ON THE ARTIFICIAL REEF
CONSTRUCTED
IN THE LONG BAY-OKURA MARINE RESERVE, AUCKLAND
11th August 2003
PERMIT
AO1/1 26th APRIL 2001
Preamble:
The aim of the research was to construct two small
patterns of artificial reef comprising different sizes of pierced concrete
hemispherical shells. These were constructed utilising the American system
called Reefballs. Colonisation of these is now being monitored. It was hoped
to extend the study to include work on sedimentation rates and the impact on
the benthic population. It is anticipated that this activity will fulfil a
range of positive outcomes:
(i)
increase
conservation awareness from the use of the Reserve as an educational resource
and recreational dive site.
(ii)
permit
future informed decisions to be made on artificial reef construction in New
Zealand.
(iii)
monitor the
problems of run off which are faced by the reserve.
Reef 1 Zig
Zag Reef:
The reef is located at 360 40 52 S, 1740
45 14.8 E in 5.5 m at minimum spring low tides. It comprises twelve concrete
hemispherical shells made of 60 MPa concrete with microsilca added. There are
nine Bay Balls [mass approximately 300 kg, height 0.7 m, base diameter 1.0 m
with about 10 piercings] in a N S line. At the Northern end, there is a
Pallet Ball [mass approximately 1200 kg height 0.9 m; base diameter 1.2m with
about 15 piercings]. ]. At the Southern end, there are two further Pallet
Balls; one with a 30x30 cm slate plate attached to a vertical surface and the
other with a plate attached to close the top hole of the unit; i.e. in a
horizontal position. These slates are the same as the ones utilised by current
E. O. S. intertidal studies. Placement took approximately 6 weeks because of
tide/weather/logistical constraints. All 12 units lie on a N S line parallel
to the beach with an angle of approximately 450 between this line
and the zigzags and about 1.0 metre between each.
Reef 2
Octagon Reef.
The reef is located at 360 40 51.1 S,
1740 45 19.6 E in 6.2 m at minimum spring low tides. This also
comprises twelve units with four of the smaller Bay Ball units planted as the
central core. There are four Bay Balls around the perimeter; a Pallet Ball at
the Eastern point of the octagon; and one further Pallet Ball on the Southern
margin. There are slate plates attached to a vertical surface of both these
Pallet Balls. Another Pallet Ball is placed on the western side. The last
Pallet Ball forms a tail to the south since inclement weather prevented it
being placed in geometrical perfection. All the spacings are again about 1.0
metre.
Colonisation
sequence:
Hard surfaces were colonised firstly by planktonic
organisms, e.g. pelagic phytoplankton. The first settlement to be observed
(after a period of about 6-7 days) was a microfilamentous brown algae
(unidentified, but common in the Gulf). The next colonising organisms were
vagrant (and opportunistic) benthos, such as the cushion star: Patiriella regularis, and the hermit
crab: Pagurus sp. c.f. P. novaezelandiae and herbivorous
gastropods like the whelk: Cominella
adspersa (egg cases of the latter were recorded on a number of the
reefballs). Concurrent with this, nektonic organisms like the
spotty: Notolabrus celidotus and
triplefins: Fosterygium varium and Fosterygium sp. feed on small invertebrates
and graze on the algae. The next wave of colonisation observed was that of
sciaphilic invertebrates such as barnacles and polychaete worms. Two
settlements of the rapidly colonising barnacle Austrominius modestus were recorded; the first occurring about ten
days after the placement of the reefballs. A second, more numerous settlement
event of A. modestus occurred about
four weeks later. Of the annelids, the first was the spiny tubeworm: Spirobranchus cariniferus which settled
about two-three weeks after reefball placement. After a month, these reached
lengths of up to 12 mm.
Both artificial reefs are close to being fully
colonised. Biodiversity on the two reefs is still somewhat low but new species
continue to colonise the reef. Most noticeable was the new settlement of the
ascidian (Sea squirt) Asterocarpa
coerulea around the sides of numerous of the Reef Balls. Larger
macro-algae, brown seaweed Carpophyllum
maschalocarpum (still at early stage approximately 5cm long) is establishing
itself around the top of some of the Reef Balls. Also identified on the zigzag
reef were two Large Trophon (Xymene
ambiguus), these are typically sandy shore species and are probably
resident in the area. A new fish
species, a juvenile Sweep (Scorpis lineolatus)
was present at the Octagon Reef. Juvenile sweep are often found on shallow
reefs, close to shelter (provided by the Reef Balls?).
The unidentified filamentous brown algae still
dominates coverage on the reef balls, however the barnacle A.modestus are now densely populated particularly on the zigzag
reef, with several settlements of barnacles evident (small and large live
shells, and older empty shells). A.modestus
is also dominating percentage coverage on the slate tiles over the spiny
tubeworm Spirobranchus cariniferus (which
was formally dominant). Numbers of Cominella
adspersa and the starfish Patiriella
regularis appear down. What appeared to be snapper divots were observed
around the surrounding reefs. There appears to be an abnormally high sedimentation
rate on the two Artificial Reefs, which may contribute to the absence of some
organisms. Further studies on sedimentation rates will hopefully follow.
May - June 2002
Zigzag Reef:
The zigzag reef is now entirely colonised. The make
up of organisms on the reef has markedly changed over the past few months. Both
biodiversity and biomass of organisms on and immediately around the reefballs
has increased, with the appearance of a host of new organisms and an increase
in numbers of some already existing organisms (personal observations). Most
noticeable was the number of fish, a few new species of which may now be
resident. The zigzag reef has seen the arrival of a number of juvenile fish
from the summer spawning season, seeking shelter and food, swimming in and
around the reef. Schools of small Snapper (less than 90mm long in schools
>50), Trevally (less than 100mm long in schools >50), Blue Maomao, and
Sweep (less than 70mm), were observed swimming around the reef, with dozens of
juvenile Goatfish (Upeneichthys lineatus),
(less than 90mm in length), pale in colour swimming along the bottom feeding on
small invertebrates in and amongst the reefballs. There were also numerous
adult Parore (Girella tricuspidata)
swimming in amongst the reefballs, some large up to 300-350mm long, some adult
Red Pig Fish. The Spotties, Triplefins, and adult Sweep are still present in
similar numbers. The triple fins have increased in size. There were many
snapper divots around the reef, and a few larger snapper (up to around 250mm)
were seen on the outskirts of the reef, at the edge of our level of visibility.
The once dominant filamentous algae has been greatly
reduced in percent coverage to a few percent only. It has been replaced with a
dense covering of barnacles (A.modestus),
tube worms (including Spirobranchus
cariniferus), and rock oysters (Saccostrea
glomerata). These are already creating second and third layers in places
colonising on top of dead and empty shells already attached to the reefballs.
There are now at least three different tubeworms
including Chaetopteros sp, and Spirobranchus cariniferus. The third species (unidentified) has a
large surface coverage on the outside of the reefballs (approx 20%). There have
been at least two different settlements of S.glomerata
with an older smaller settlement of oysters up to 40mm long (1-2% coverage)
and a more numerous recent settlement of oysters up to 20mm long (approx 10%
coverage). The oysters are more prominent on the inside cavity walls and hole
walls of the reefballs than the outside.
There is a small number of juvenile green-lipped
mussels (Perna caniculus),
approximately 2 cm long, establishing themselves in the base of the side hole
walls. New gastropods to the zigzag reef include the Siphon Whelk (Penion sulcatus), the Spotted Whelk (Cominella maculosa), and the Oyster
Borer (Lepsiella scobina) which are
all carnivorous and were located on the outside of the reefballs.
Brown macro-algae up to 10cm long (possibly Halopteris novae-zelandiae) is
establishing itself within the holes of the reefballs with at least two species
of sponges also are colonising the outside of reefballs. Dozens of small
shrimps are to be seen seeking shelter in small cracks underneath the
reefballs.
The population of cushion stars is noticeably down
from previous dives, with an increase of a larger starfish, Coscinasterias calamaria (up to 200mm)
resting in the holes of the reefballs and on inside cavity walls.
Colonisation on the
octagon reef is not yet complete at around 80-90%. The colonisation makeup is very similar to that of the zigzag
reef with a few differences, most notably the difference in coverage of
barnacles and oysters. Very few oysters and barnacles have colonised the
octagon reef in comparison to the zigzag reef. The filamentous algae is still
present though in increasingly smaller coverage but the tubeworms have a much
greater dominance (S,cariniferis, and
the unidentified species). There are
similar numbers and species of juvenile fish present, with various schools
sheltering within the octagon.
December 2002
Zigzag Reef
Bottom Conditions: The seafloor and Reefballs had a thick blanket of a fine silt/clay
deposit. The silt/clay was much more pronounced than on previous dives, but
could in part be attributed to the high rainfall experienced in the region over
the past few days, and the close proximity to the Vaughn Stream and Okura
River. This layer is the greatest experienced so far (personal observation)
since January 2000 making visibility extremely low (0.5m-2m). The sediment was
gelatinous and over 20 cm deep in the area around the reef.
Whilst the Reefballs still
sit relatively flush with the seafloor, scouring around the outside of the
Reefballs has continued and the entire artificial reef is now approximately
20cm below the seafloor norm. Even greater scouring has occurred between the
Reefballs, in some instances lateral movement is occurring with the Reefballs
subsiding and slipping together. The gaps between some of the Reefballs (initially
>1m) have been reduced to as little as 20cm.
Colonisation: The reef is close to being fully colonised with
biodiversity somewhat lower than in August 2002 (NB: Because of the low
visibility and thick layer of sediment blanketing the Reefballs some species
may have been missed). Visibility inside the Reefballs was close to zero, hence
accurately detailing colonisation on the inside surface was difficult.
Algae/Seaweed: The brown filamentous algae has returned and covers
close to 80% of the Reefballs, growing on existing sciaphilic and other
organisms. There have been some new settlements of the green seaweed Codium fragile and the brown seaweed Halopteris novae-zelandiae, they seem to
have become relatively well established reaching lengths upwards of 8cm (C.fragile) and 6cm (Halopteris novae-zelandiae?). The plant numbers are still
relatively low (1-2 of each species per RB), with the majority located on the
top surface of the Reefballs, and around the side holes. Low numbers of seaweed
is probably due to a number of factors, including grazing by invertebrate and
fish species.
Echinoderms: Numbers of both the Patiriella regularis and Coscinasterias
calamaria were well down on numbers found in August 2002. There were a few P.regularis on the outside of the
Reefball and no sighted C.calamaria.
Sciaphilic Organisms: With the silt/clay mixture cleared, the Reefballs
reveal a large number of dead/empty shell remains. Many of these are already
being colonised by spat settlements of other sciaphilic organisms. There seems
to be a high turnover rate with regular spat settlements of a number of
different organisms, (namely the barnacle Balanus
trigonis, pacific oyster Saccostrea
glomerata, and tubeworm species including Spirobranchus cariniferus) of which there are a number of different
population sizes. There are few living larger older specimens though many dead
shell remains of this cohort, (the cause of which is probably a mixture of
predation, competition, and smothering from silt and clay deposits), with a large
number of living small juvenile species. The number of live oysters on the
outside surface of Reefballs has been drastically reduced, in saying this, on
previous dives most oysters had colonised the inside surface of Reefballs which
was difficult to see with such a low visibility. It appears only the hardy
species are surviving to maturity and adulthood, with a lot of casualties of
the more sensitive species, and a high turnover of animals. The barnacle (B.trigonis) is out competing most of the
tubeworm populations. There were a couple of new unidentified calcareous
tubeworm species.
Other organisms: The most prominent species on the Reefballs was the
spotted whelk (Cominella maculosa),
there were well over a hundred on many of the Reefballs, grouped together with
egg sacks littered around the outer Reefball surface. These ranged in size up
to 4cm (Similar results were observed this time last year).
The
sea squirt (Ascidian) population
common during the last dive has been largely reduced to a few individual
specimens of a small size (5cm length). The remaining few were heavily covered
in algal (filamentous algae) and sediment deposits.
There
were no sponges seen on this dive.
There
were some large specimens of the siphon whelk (Penion sulcatus) up to 9cm, along with a few attached balls of egg
capsules.
The
mussel (Perna caniculus) has a
stronghold on the bottom inner surface walls of the Reefball holes where they
are growing in small clusters. Some have grown in size to 5cm or more in
length.
The
Turret shell (Maoricolpus roseus,
small to 2cm) was present in small numbers in the hole walls.
The sea anemone numbers
were well down, most Reefballs had none, a few small populations have survived
on a couple of the pallet balls.
Fish Species: Because of the low visibility the number of fish counts were extremely
low. Those seen include the Spotty, and the three Triplefin species. Numbers
were obviously low due to the visibility. There was evidence of snapper and
stingray/eagleray activity with numerous divots around the Reefball site. Large
numbers of the Hermit Crab (Pagurus sp.
c.f. P. novaezelandiae) littered the
outer walls of the Reefballs.
Slate Tiles:
Colonisation still dominated by the barnacles B.trigonis and A.modestus,
up to 80%. The were no Ascidians found on the slates. The tubeworms
(particularly Chaetopteros sp.) made from, 10-15% coverage, with bare rock the
remaining few percent. One of the slates (the slate tile covering the top
central hole) had a 7cm Codium fragile
plant growing from it. There was little difference in coverage and in species
between the 3 different slates.
The Octagon Reef
Bottom Conditions: A similar >20cm deep gelatinous silt/clay layer on seafloor, and
blanketing the Reefballs. Conditions are more silty/muddy than the Zigzag Reef.
Colonisation is at best at around 80% with the concrete visible when silt
cleared away. Biodiversity is similar to that of the Zigzag Reef with
barnacles, tubeworms the dominant species. The oyster populations have
disappeared with only a few live specimens observed (plenty of dead shell
remains). There are now no mussels within the hole walls (Predation by
octopus??). The resident octopus was not observed!
February 2003
Zigzag Reef
Bottom Conditions:
·
Substrate: fine
sand/shell, 1 2cm layer of clay/silt also covering Reef.
·
Visibility: 4-5m
·
Reef Ball positioning:
Scouring around reef to 30cm deep extending 2-3m either side of reef, Reef
still flush with seafloor.
·
Minor lateral Reef Ball
movement due to deeper scouring between Reef Balls, with two Bay Balls in
Zigzag Reef now only 30cm apart.
Colonisation:
Algae/Seaweed: Filamentous algae much reduced. Good algal growth of brown seaweed Melanthalia abscissa. Most plants young
up to 3-4cm in length, irregularly dotted over Reef Balls there are a few older
specimens up to 15cm in length. Some large specimens of the green seaweed Codium fragile are present ranging in
size from 4 to 16cm in length, most common on top of Reef Balls. There is a new
colonisation of unidentified red seaweed in small numbers to 4cm.
Sessile Invertebrates: Heavy colonisation of Perna caniculus (Green lipped mussel) in clumps particularly around
piercings and base of Reef Balls ranging in size up to 8cm. Saccostrea glomerata (Pacific Oyster)
has good coverage on outer and inner surfaces of Reef Balls, generally small in
size to 3cm. High majority coverage of the two barnacles (Austrominius modestus, Balanus trigonis), have generally displaced
much of the tubeworm populations (Spirobranchus
cariniferus, Chaetopteros sp, Pomatoceros terranovae) though these
tubeworms are still present in small numbers. The encrusting sponge (Cliona celata) growth is irregular they
are more common around piercings and outer
surface of Reef Balls and range in size to 20cm
length. A few Golf Ball sponges are present around outside of Reef Balls to 2cm
in diameter.
A new ascidian (Asterocarpa coerula) is present with populations
up to 2-3cm in length, a few older ascidians to 9cm in length and densities
relatively low at an average of around 7-8 per Reef Ball.
Good numbers of sea
anemones (Anthopleura aureoradiata)
were seen, most in dense congregated populations, with a few scattered isolated
specimens.
Vagrant Benthos: Hermit crabs (Pagurus
novaezelandiae) common in piercings of Reef Balls and around the outer
surfaces. Spotted whelks (Cominella
adspersa) in small numbers only.
Small numbers of Patiriella
regularis (av of 1-2 per Reef Ball) and Coscinasteris
muricata (3 seen on Reef) in and amongst the Reef Balls.
Fish: A
lot of fish were seen including adult sweep 20-25cm; adult Parore 35cm;
Goatfish adult and juvenile 15-30cm; shoals of 20 to 50 juveniles 5-7cm long
probably Koheru. Numerous triplefins including both common and variable and
also, possibly, estuarine. These ranged from 3cm juveniles to 12cm adults.
Small black [spawning males?] about 4cm long were present at both reefs with
half a dozen individuals on each Reefball. Numerous Spotties ranging from 10
25cm and Snapper [30cm?] lurking at the edge of visibility. Large divots are
abundant away from the reefs, some 60cm across and 30 cm deep.
Octagon Reef
Bottom Conditions:
·
Substrate: fine
sand/shell, 1 2 cm layer of clay/silt. Reefballs heavily covered in silty
sludge.
·
Visibility: 1-2m, a lot
of suspended material.
·
Reef Ball positioning:
Scouring to 30cm deep extending 2-3m either side of reef, Reef still flush with
seafloor.
·
Minor lateral Reef Ball
movement.
Colonisation General: Colonisation and marine life on the Octagon Reef is
much lower in comparison to the Zigzag Reef. The silt sludge layer on the
Octagon will have smothered a lot of the previous existing life and prevented a
lot of new growth.
Algal growth:
Filamentous algae much reduced. There is some new growth of the brown seaweed Melanthalia abscissa. Most plants small
up to 3-4cm in length, irregularly dotted over Reef Balls, much fewer plants in
comparison to the zigzag reef. A couple of older specimens on the reef present,
up to 10cm in length.
A few specimens of the
green seaweed Codium fragile, ranging
in size up to 10cm in length, most common on top of Reef Balls, with a new
colonisation of unidentified red seaweed in small numbers to 4cm.
Sessile Invertebrates: A few isolated mussels present around the Reef Ball
piercings, up to 6cm in length. Numbers much less than on zigzag reef. Lots of
dead Pacific Oyster shell under silt layer with only a few live specimens.
Colonisation on outer surface of Reef Ball dominated by barnacles (A.modestus, B.trigonis) and tube worms (S.cariniferous, P.terranovoe). Numerous
Golf Ball sponges on outer surface of Reef Balls with some colonisation of
encrusting sponge (Clione celata),
with 2-3cm diameter.
Vagrant Benthos: Small numbers of Patiriella
regularis and Coscinaster muricata
on and around Reef Balls
Fish: Fewer fish species seen, perhaps
because of low visibility. Spotties, small schools of Koheru (3-4cm in length)
and a few triplefin fish (same varieties but in lower numbers to Zigzag reef)
were sighted.
Overall change in biota:
New organisms are present in the area. These have
either migrated from other parts of the marine reserve, or have settled from
the moving water body. (A combination of both is likely). The latter group
probably would not have colonised the area if the reefballs had not been present. It is very encouraging to observe such large
numbers of juveniles present at times.
It is also impressive how resilient the reef biota proved after an
enormous silt dump in Spring 2002.
Comparison of surfaces:
No significant differences in the colonisation of the
smooth areas of concrete surface, rough aggregate surfaces nor the slate plates
have been noted.
Bottom Conditions:
Sediment: The sites selected have bottom sediments classified as grey-brown shelly muddy fine sand. Sediments around the reefballs contain a larger amount of fine silt than could be expected in this environment. The origin of this is probably urban run-off. Increasing rates of development locally will undoubtedly increase this. There are large numbers of empty bivalve shells on/near the surface.
Two trial sedimentation collectors (made from a PVC
pipe 100mm in diameter and 700mm long, staked to the seafloor) have been tested
around the octagon reef in attempt to study the rates of sedimentation. Results
of these trials were inconclusive since there was not possible to establish a
regular monitoring programme.
Settlement: Although there is a significant depression some 30
cm deep scoured around both reefs, the individual Reefballs still show no
appreciable settlement into the sediment. The holes in the base of the units
were left open and no additional thickening measures taken in manufacture.
There have been many storms over the period with onshore winds gusting up to
100 kmh-1 however the site selected appears to be well protected,
with the artificial reef remaining stable.
Concrete pH Tests
Tests of the
surface pH of the well-weathered units stored at Gibbons Crib Walls show that
the surface pH lies in the range 8 9. Technical difficulties with the AUT
Datasonde have prevented any meaningful results being taken from the reefballs
in the water as yet. The lack of difference in colonisation between the
available surfaces tends to indicate that the pH range of the concrete
substrate is satisfactory.
Project team:-
|
Supervisor:- Professor John Buckeridge
|
Jonathan Jaffrey
Trent Taylor |
Earth and Oceanic Science
Research Centre Faculty of Science and Engineering
|
|
Paul
Murphy |
Auckland University of
Technology
|