Biomass

The A. nodosum resource has been divided into six geographic areas along the Nova Scotia and southern New Brunswick shores (Fig. 3). Acadian Seaplants Limited (ASL), a Canadian seaweed processing company, has been granted 11 leases in NS in the most productive rockweed areas, and the whole of the southern NB coastline. The area corresponds to 76% of the total resource for the region. For management purposes, the company has divided its leases into 340 harvesting sectors (Ugarte and Sharp, 2001) and has been assessing its resource annually since 1995 using a combination of aerial photography and extensive ground truthing. Thus, the standing stock and general condition of the resource, including its

Figure 3. Ascophyllum nodosum distribution in Nova Scotia and southern New Brunswick. Darkest areas along the coastlines indicate zones having higher concentrations of biomass. (SWNS: Southwestern Nova Scotia).

population structure are well documented over most of the commercially valuable sites. The uncertainty in these values exists in area 5 of Nova Scotia, a relatively unproductive area where biomass surveys have been less intense.

The standing crop of summer (July-August) biomass for this region has been calculated at 353,395 wet tons, covering an area of close to 4,960 ha (G. Sharp, 2009, personal communication; R. Ugarte, unpublished data), to give an average biomass density of 71.3 t ha-1. The highest A. nodosum abundance is concentrated in Lobster Bay (area 3) in southwestern Nova Scotia (SWNS) and southern New Brunswick (area 6) with a total of 91,758 and 158,811 t, respectively, corresponding to 71% of the total resource biomass (Fig. 3; Table 1). Biomass densities range between 86 and 87 t ha-1 in these areas.

2.2. HARVEST

Commercial exploitation of rockweed along the coastal areas of Nova Scotia began in the late 1950s when it was used as a raw material for manufacturing sodium alginate and "kelp" meal. Today, this seaweed is used as a source of a fertilizer extract and as an animal feed supplement. It is the main economic resource of the seaweed industry in the Maritime Provinces and of the country. Rock-weed harvest in the region reached peak landings in 2007 and 2008 with just over 36,500 t, with 86% of the total landings corresponding to areas 3 and 6 (20,061 and 11,303 t, respectively) (Table 1). Although the total landings represent only 10.3% of the standing biomass (Table 1), the harvest in the region, with the exception of NB, is considered to be in a fully exploited condition because only 2,118 ha or 42.7% of the resource is actually accessible (Table 1). The remaining areas are either too exposed to the weather or waves, or the biomass density is too low to be profitable for the harvesters and the industry. Thus, the harvest yield varies between 13.6 and 20.5 t ha-2 a-1 in the accessible portion of the resource (Table 1). Area 3 is the most productive and has maintained this yield level since 2001. It appears that all the currently harvested areas in NS have reached or are

Table 1. Total rockweed biomass and landings in the rockweed areas of Nova Scotia and New Brunswick.

Area covered

Standing stock

Accessible area

Landings 2008

Yield

Rockweed area

(ha)

(wet tons)

(ha)

(wet tons)

(t ha-1)

1

35

998

35

500

14.3

2

93

9,031

47

844

18.0

3

1,073

91,758

977

20,061

20.5

4

677

42,125

229

3,839

16.8

5

1,250

50,000

0

0

0.0

6

1,832

158,811

830

11,303

13.6

Total

4,960

352,723

2,118

36,547

very close to their maximum annual sustainable yield. Area 6 in NB could potentially increase its yield as it is currently under a fixed exploitation rate of 17% (Ugarte and Sharp, 2001).

2.2.1. Production, Productivity, and Carbon Fixation

Annual production values for A. nodosum for areas with different exposures to wave action in the Maritimes were estimated by Cousens (1981, 1984). His P/B (Productiv-ity/Biomass) values varied from 0.22 to 0.79. Considering these estimates and our own observations of the distribution of the resource in the region, a P/B average of 0.54 seems reasonable. Thus, based on summer estimates of biomass, the total annual rockweed production for the region is 54,055 dry tons, equivalent to 20,000 t of carbon annually. This production would require the net absorption of 73,284 t of CO2 from the environment each year (Table 2). Areas 6 and 3 obviously are contributing the bulk of the rockweed production and CO2 absorption (Table 2). Our analysis shows that the highest production is in area 2 in Saint Mary's Bay with 561.5 g C m-2 a-1 and the lowest production is in area 5 (Eastern Shore) with 231.8 g C m-2 a-1 (Table 2). The annual estimates of Ascophyllum productivity (232-562 g C m-2 a-1) observed in the Maritime Provinces are slightly below the range (300-894 g C m-2 a-1 1) estimated for this species for the Northwest Atlantic (Mann, 1973; Brinkhuis, 1977; Cousens, 1981, 1984; Roman et al., 1990; Vadas et al., 2004). These earlier studies considered the spring biomass, a time when the deciduous receptacles of A. nodosum are most numerous and largest in size. Our results represent larger-scale measurements and should serve as a base for future comparisons during summer.

Further, estimates of carbon sequestered in the standing biomass depend on the composition of the seaweed at the time of harvest. First, the ratio of dry mass to fresh biomass must be considered. Ascophyllum nodosum growing in regions of significant currents and upwelling water are subjected to elevated nutrient levels, which favor a low ratio of dry to live biomass. We encounter this routinely for A. nodosum harvested from the lower Fundy region (area 6) when compared with biomass from the open NS coastline (areas 3-5). However, embayments with low flushing rates can suffer from reduced nutrient availability in the summer seawater and this leads to elevated dry to live biomass ratios. We have used a 29% conversion for areas 1-5 and 27.5% for area 6 (Table 2).

The composition of the A. nodosum must be considered, as it is known to vary significantly throughout the annual growth cycle. Analyses for A. nodosum provide a range of values for the major components of the seaweed (Indergaard and Minsaas, 1991). We have used an average of the values reported for the seaweed, and computed the carbon content for each major component to arrive at the 37% average overall carbon content for dry A. nodosum. Because the seaweed harvests occurred in summer, a period of major carbohydrate and dry matter accumulation, the use of an average carbon content may slightly underestimate the quantity of net carbon sequestered. In this regard, Vinogradov (1953; Table 10, p. 27) gives 37.99% as the carbon content for A. nodosum.

Table 2. Standing stock, production, and productivity values of Ascophyllum nodosum in the Canadian Maritime Provinces.

Rockweed

Area covered

Standing stock

Production

Productivity

Productivity

CO2 conversion

area

(ha)

(dry tons)

(dry tons a-

-1) (t C a-1)

(g C m-2 a-1)

(t CO2 a-1)

1

35

484

262

97

276.5

355

2

93

2,619

1,414

523

561.5

1,917

3

1,073

26,610

14,369

5,317

495.5

19,481

4

677

12,216

6,597

2,441

360.5

8,944

5

1,250

14,500

7,830

2,897

231.8

10,615

6

1,832

43,673

23,583

8,726

476.3

31,973

Totals

4,960

100,102

54,055

20,000

2,402

73,284

Production values assume an average P/B = 0.54 for the Maritime Provinces (Cousens, 1981). A carbon content of 37% of the seaweed dry mass was used in the calculations, and the total C x 3.664 gave the net amount of CO2 assimilated.

Production values assume an average P/B = 0.54 for the Maritime Provinces (Cousens, 1981). A carbon content of 37% of the seaweed dry mass was used in the calculations, and the total C x 3.664 gave the net amount of CO2 assimilated.

2.3. CURRENT CONDITION OF THE RESOURCE

The rockweed resource of the Maritime Provinces has been observed and studied for almost 80 years (MacFarlane, 1931-1932, 1952; Sharp, 1981; Cousens, 1981, 1982, 1984; Ang et al., 1993, Ugarte et al., 2008). The condition of the resource was considered stable until the early 2000s when changes started to become apparent. Specifically, these changes have been in the form of bed damage due to unusual ice patterns, increased abundance of Fucus vesiculosus, and massive mussel recruitment in the intertidal zone.

2.4. ICE DAMAGE

Severe ice damage was observed in 2003 and 2004 in several harvesting sectors from areas 1-4 in NS (Fig. 3). In 2000, the ice pattern in the Gulf of Saint Lawrence Region began to change with a reduced period of ice cover during that year, a drastic change after that, and a historically record low in 2006 (Fig. 4). That change in ice cover corresponded well with an increase in the SST during that period for SWNS, which, in turn, corresponded to the generally increased air temperature in SWNS for the same period (Fig. 5). Rapid increases in air and SST during the spring of 2003 and 2004 produced ice breakup into rather larger pieces that scoured the intertidal zone. This scraping effect was exacerbated by early season southwest winds that normally arrive in late spring and pushed ice against the shore. Several A. nodosum harvesting sectors exposed to the southwest wind were considerably damaged, with some losing up to 90% of the available biomass.

Ice damage in the upper intertidal zone is a common phenomenon along the Atlantic coast of NS that, on occasion, has resulted in considerable damage to the intertidal zones of sites such as Saint Margaret Bay, near Halifax (McCook and in

200 -, 180 -160 -140 -120 -100 -80 -60 -40 -20 -0 -

^ ^ ^ ^ ¿f ¿f ^ ^ J" ^ ^ J ^ ^ ^ ^ ^ ^ J? ^ f ^

Figure 4. Ice duration in the Gulf of Saint Lawrence Region, 1963-2007. Arrow indicates the lowest ice duration in more than 4 decades (Data provided by Joel Chasse, DFO, Canada).

Figure 5. Annual mean air temperature (solid line) observed for southwestern NS (Yarmouth station) from 1970 to 2007 and winter (December-March) surface seawater temperatures (SST) (triangles) observed for the same area in the early 1990s and from 2000 to 2007.

Chapman, 1991). However, the magnitude of the damage observed for two consecutive years in the ASL leases has not been observed before by the local harvesters of southwestern NS, some of whom have harvested in this area for more than 35 years (J. Brennan, 2004, personal communication). In fact, during the winter of 2007-2008 and for the first time in living memory, the bays of south-western NS did not freeze over (J. Brennan, 2008, personal communication; R. Ugarte, 2008, personal observation).

2.5. INCREASE OF FUCUS VESICULOSUS

Historically, the rockweed beds of southwestern Nova Scotia have been composed of almost 99% A. nodosum, with a minor component of F. vesiculosus (G. Sharp, 2008, personal communication). The incidence of F. vesiculosus in the harvested material has been extensively monitored by ASL since 2003, after observing a slight increase from the previous two years. From an average incidence of 0.8% prior to 2003, it increased almost steadily each year to 4.6% in 2008 (approximately 500% increase). Field surveys confirmed this trend for 2005-2007 (Fig. 6). The sectors showing the highest incidence of F. vesiculosus were located mostly in protected areas such as Lobster Bay and the inner sectors of Saint Mary's Bay (Fig. 3).

It is tempting to attribute the increase in F. vesiculosus incidence to the harvest, especially since the total landings have steadily increased to a peak of over 36,500 t in 2008. However, the increased landings were the result of the opening of new areas or leases rather than increased exploitation rates within the existing harvesting areas. Some sectors in Lobster Bay (area 2) have been harvested at the same exploitation rate or higher since the early 1970s (Sharp, 1986) with no increased occurrence of F. vesiculosus until very recent years. The specially designed rake used by the harvesters has been proved to cut only a portion of the A. nodosum fronds without stripping the plants from the substratum (Ugarte and

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