440
Chapter 11: The Pacific Ocean
here, strongly influenced by the Petersen school of thought, located at least three coastal
community types that could be accommodated in the global system of isocommunities—
and even if this was a strictly subjective judgment, it was not lacking in useful information.
Thus, Thorson identified a regional version of the Tellina isocommunity, dominated in
this case by T. liliacea, on clean sandy bottoms in the sublittoral; deeper off the open
coasts of the South Island, on soft muddy grounds, an Amphiura isocommunity was
found to occur. This was dominated by Amphiura rosea, with Echinocardium, Nucula,
Dosinia, and polychaetes—Glycera, Lumriconeries, and Pectinaria. In some other regions
this assemblage is dominated by one of its components, Turritella; here, the Turritella-like
Maoricolpus dominates in some regions, as in deep fjords.
Recent investigations have looked at larger areas and used less subjective techniques,
but their results appear compatible. Studies that have examined the grouping of biota
across the entire width of the shelf by numerical classificatory techniques have identified a
small number of depth-specific communities of benthic invertebrates, of which the most
remarkable is perhaps the midshelf community off the Otago peninsula dominated by
bryozoans (Filicea elegans) on a substrate of pebbles, with little silt deposition. Otherwise,
three or four depth-specific, substrate-specific assemblages are recognized with the use of
numerical classification (Probert and Wilson, 1984; Probert and Grove, 1998); polychaetes
dominate numerically at most stations, in most assemblages. Thus, off the west coast
of South Island, the assemblages identified by such techniques were (i) on silty sand at
30–50 m, a grouping dominated by Sthenelais, Nephthys, Ampharete, and others, (ii) at
90–300 m on sandy mud characterized by bivalves (Poroleda) and amphipods (Ampelisca)
and polychaetes (Aglaophamus, Lumbrinereis and others), (iii) on sand at 200–250 m
characterized by bryozoans and Chloeia, another polychaete, and (iv) at >500 m on sandy
mud characterized by tanaids and ophiuroids.
A trawling survey off southern New Zealand likewise specified demersal fish assemblages that could be related to characteristic depth zones, but not to sediment type because
the range of types was too narrow. Shallow assemblages were dominated by Gempylidae,
Squalidae, and Moridae, whereas deeper on the shelf Chimaeridae, Argentidae, Merluccidae, and Macrouridae were characteristic. Off the northeast coast and down to at least
Cook Strait, subtropical families dominate: breams (Sparidae) are very characteristic.
More generally, the overall diversity of demersal fish communities off New Zealand from
many such surveys has been related to depth, latitude, and regional phytoplankton productivity (McClatchie et al., 1997): species richness decreases with latitude and increases
with depth and surface productivity. It is no surprise, therefore, that diversity of demersal
fish is maximal between the 500- and 1000-m contours on Chatham Rise; it was thought
that the relatively high degree of mesoscale activity over the northern slopes of this feature
adjacent to the New Zealand shelf is responsible for the large number of species-rich
locations found here on the steeply sloping slope. However, perhaps it is not as simple
as it seems.
This is, in fact, a beautiful example with which to finish this chapter, to warn us of
the dangers inherent in the subjective relationship that I have proposed, or reported on,
throughout this book. For, having offered the relationship just expressed, the authors
warn that perhaps a quite different relationship may actually determine relative diversity.
The greatest diversity, found on steep slopes on the Chatham Rise, may, they suggest,
have more to do with the fractal nature of this habitat than the overlying productivity
that itself may be provoked by the same topographic roughness that is likely to induce
mesoscale eddies and productivity above. They also point out that diversity may be low
in the higher latitudes surveyed simply because of lack of rough habitat there. Although
I think it is not their favored interpretation, they point out that the association between
fish diversity and surface productivity may be correlative but not linked by causation.
Would that more authors understood that possibility.
Chapter 11: The Pacific Ocean
here, strongly influenced by the Petersen school of thought, located at least three coastal
community types that could be accommodated in the global system of isocommunities—
and even if this was a strictly subjective judgment, it was not lacking in useful information.
Thus, Thorson identified a regional version of the Tellina isocommunity, dominated in
this case by T. liliacea, on clean sandy bottoms in the sublittoral; deeper off the open
coasts of the South Island, on soft muddy grounds, an Amphiura isocommunity was
found to occur. This was dominated by Amphiura rosea, with Echinocardium, Nucula,
Dosinia, and polychaetes—Glycera, Lumriconeries, and Pectinaria. In some other regions
this assemblage is dominated by one of its components, Turritella; here, the Turritella-like
Maoricolpus dominates in some regions, as in deep fjords.
Recent investigations have looked at larger areas and used less subjective techniques,
but their results appear compatible. Studies that have examined the grouping of biota
across the entire width of the shelf by numerical classificatory techniques have identified a
small number of depth-specific communities of benthic invertebrates, of which the most
remarkable is perhaps the midshelf community off the Otago peninsula dominated by
bryozoans (Filicea elegans) on a substrate of pebbles, with little silt deposition. Otherwise,
three or four depth-specific, substrate-specific assemblages are recognized with the use of
numerical classification (Probert and Wilson, 1984; Probert and Grove, 1998); polychaetes
dominate numerically at most stations, in most assemblages. Thus, off the west coast
of South Island, the assemblages identified by such techniques were (i) on silty sand at
30–50 m, a grouping dominated by Sthenelais, Nephthys, Ampharete, and others, (ii) at
90–300 m on sandy mud characterized by bivalves (Poroleda) and amphipods (Ampelisca)
and polychaetes (Aglaophamus, Lumbrinereis and others), (iii) on sand at 200–250 m
characterized by bryozoans and Chloeia, another polychaete, and (iv) at >500 m on sandy
mud characterized by tanaids and ophiuroids.
A trawling survey off southern New Zealand likewise specified demersal fish assemblages that could be related to characteristic depth zones, but not to sediment type because
the range of types was too narrow. Shallow assemblages were dominated by Gempylidae,
Squalidae, and Moridae, whereas deeper on the shelf Chimaeridae, Argentidae, Merluccidae, and Macrouridae were characteristic. Off the northeast coast and down to at least
Cook Strait, subtropical families dominate: breams (Sparidae) are very characteristic.
More generally, the overall diversity of demersal fish communities off New Zealand from
many such surveys has been related to depth, latitude, and regional phytoplankton productivity (McClatchie et al., 1997): species richness decreases with latitude and increases
with depth and surface productivity. It is no surprise, therefore, that diversity of demersal
fish is maximal between the 500- and 1000-m contours on Chatham Rise; it was thought
that the relatively high degree of mesoscale activity over the northern slopes of this feature
adjacent to the New Zealand shelf is responsible for the large number of species-rich
locations found here on the steeply sloping slope. However, perhaps it is not as simple
as it seems.
This is, in fact, a beautiful example with which to finish this chapter, to warn us of
the dangers inherent in the subjective relationship that I have proposed, or reported on,
throughout this book. For, having offered the relationship just expressed, the authors
warn that perhaps a quite different relationship may actually determine relative diversity.
The greatest diversity, found on steep slopes on the Chatham Rise, may, they suggest,
have more to do with the fractal nature of this habitat than the overlying productivity
that itself may be provoked by the same topographic roughness that is likely to induce
mesoscale eddies and productivity above. They also point out that diversity may be low
in the higher latitudes surveyed simply because of lack of rough habitat there. Although
I think it is not their favored interpretation, they point out that the association between
fish diversity and surface productivity may be correlative but not linked by causation.
Would that more authors understood that possibility.