Transect Observations ’69 & ’94

NAVIGATION
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DRAFTING… Contents from a draft manuscript, 1999.

Fig 1

Fig 2

Seagrass beds at Bath occur partially in the lee of algal- and coral-encrusted rocks rising to MLW level and above lying at 150-300 m from shore (Fig. 1). The beds have a complex topography and distribution of sediments as a result of the irregular topography of the bedrock, the complex wave regime, and prolific biogenic production of particles of clay to cobble size. The seagrass bed is shallow (mostly less than 1 m) and conditions are generally turbulent except for a few hours at low water; during the winter months, waves over the seagrass bed may reach one meter in height and Iongshore currents speeds in excess of 60 cm/sec (Patriquin 1975).

Four seagrass substrate types with well defined distribution were distinguishable in 1969 (Fig 2). These are

(i) Cobble Framework: T. testudinum-stabilized accumulations of cobble-sized rhodiles and coral debris at the seaward fringe of the beds;
(ii) Porites Rubble Flats: T. testudinum– and H. wrightii-stabilized accumulations of P. furcata debris rising to close to MLW nearshore which alternate with
(iii) Predominantly Sand: T. testudinum– and S. filiforme-colonized predominantly sand substrates;  and
(iv) Cobble Sand: a mixed cobble-sand substrate stabilized by T. testudinum andS. filiforme occurring through the rest of the area (Fig. 2).

In Fig 3, topographic profiles, the distribution of  seagrasses, Porites furcata and Avrainvillea spp and the occurrence of Cobble Framework substrate in 1969 and 1994 are presented for the two transects (M and S4 in Fig 2) which could be most accurately relocated in 1994.

In 1994, the Cobble Framework accumulations had largely disappeared from the seaward fringe of Transect M, the depth increased and the continuous seagrass cover of that area was broken up into patches (Fig. 3, M: compare profiles for 1969 and 1994 at 120 to 170 m; hatching shows the volume lost 1969 to 1994).

In 1994, Cobble Framework substrates, and overall, much more coarse rubble were also observed shoreward of where the Cobble Framework had been in 1969 (Fig. 2, Transect M). Large areas of this Cobble Framework or mixed Cobble-Sand substrates were overgrown and stabilized by a carpet-like cover of the green algae Avrainvillea rawsonii and A. nigricans (Fug 5), which had been seen in only a few more restricted areas in 1969.

An erosional scarp in the region of Station C (Fig 2)  revealed 10 t0 30 cm of rubble including rhodiles on top of Cobble-Sand substrate.

On transect 4S, there was an overall downward slope seaward (Fig. Profle 4S). In 1969, a bed of cobbles not exhibiting distinct relief, occurred at 2-3 meters depth over a distance of 20 m.  In 1994, an area of Cobble Framework with distinct relief occurred shoreward of this area, and there was an overall pattern of greater relief and occurrence of cobble-sized materials towards shore.

These patterns suggest that a major storm event disrupted the cobble accumulations and moved much of that material closer to shore. I asked an elderly local resident whether he was aware of any major disruptions of this sort in recent years. He said that in the 1980s, wave action during a hurricane had deposited massive amounts of coral debris on shore one day, and then took it back to sea on the next day; it was the most severe storm disturbance of the Bath area that he could remember.

Discussion