abundance and diversity of seagrass and macrofauna in the intertidal areas with and without seaweed farming activities in the east coast of Zanzibar

The diversity and abundance of seagrass and associated macrofauna were studied in transects with and without seaweed farms at Chwaka Bay and Jambiani, in the East Coast of Zanzibar. Eight seagrass species, namely Cymodocea rotundata, Cymodocea serrulata, Thalassia hemprichii,Thalassodendron ciliatum, Syringodium isoetifolium, Halodule uninervis, Halophila ovalis and Enhalus acoroides were recorded in the transects. The mean total biomass of seagrass at Chwaka Bay ranged from 142.4 ± 70.71 to 1652 ± 772.7 g dw/m2 and 212.9 ± 146.2 to 1829 ± 1692 gdw/m2 in station with and without seaweed farms, respectively. At Jambiani, the mean total biomass ranged from 880.4 ± 336.8 to 3467 ± 549.9 and 203.4 ± 102.4 to 3810 ± 2770 g dw/m2 in station with and without seaweed farms, respectively. The overall total biomass of seagrasswas significantly lower (KW = 108.7, p < 0.0001) in station with seaweed farms than in stations without seaweed farms. A total of 93 macrofauna species representing 60 families were encountered and the mean density ranged from 910 to 6990 individuals/m2 at Chwaka Bay andJambiani in stations with and without seaweed farms respectively. The most common macrofauna species were Codakia punctata, Meropesta nicobarica, Echinometra mathaei, Pinna muricata and Clibanarius emystemus. It was shown that the macrofauna abundance and diversity was higher in stations without seaweed farms than in the stations with seaweed farms, which could be due to activities associated with seaweed farming which contributed to the loss of diversity and biomass of flora and macrofauna of the seagrass meadows

Photosynthetic performance, epiphyte biomass and nutrient content of two seagrass species in two areas with different level of nutrients along the Dar es Salaam coast

Heavy nutrient loads in coastal waters often lead to excessive growth of microalgal and macroalgal epiphytes on seagrass leaves, with varying effects on the underlying seagrasses. This study evaluates the photosynthetic performance, epiphytic biomass and tissue nutrient content of two tropical seagrasses, Cymodocea serrulata and Thalassia hemprichii, in two intertidal areas along the Dar es Salaam coast in the Indian Ocean, a nutrient-rich region at Ocean Road (near the city centre), and a nutrient-poor region at Mjimwema (south of the city centre). Epiphyte biomass was significantly higher at the nutrient-rich site, and epiphytes were associated with reduced photosynthetic performance in both seagrass species at both sites. Likewise, nitrogen and phosphorus tissue content was higher in both species at the nutrient-rich site than at the nutrient-poor site. Epiphytic species composition on the seagrass leaves varied between seagrass species and between sites. Cymodocea serrulata had a higher number of epiphytic species at Mjimwema than at Ocean Road, whereas Thalassia hemprichii had more epiphytic species at Ocean Road than at Mjimwema. Seagrass photosynthetic performance, epiphytic biomass and nutrient content of the seagrasses were shown to be affected by nutrient concentration in the water column. Thus, for the future monitoring of the seagrass meadow, we recommend the use of combined measures such as seagrass performance, epiphytic biomass, nutrient contents and nutrient concentration levels in the water column.Keywords: C:N:P ratio, Cymodocea serrulata, photosynthetic activity, Thalassia hemprichiiAfrican Journal of Marine Science 2012, 34(3): 323&#8211;33

Seagrass Resistance to Light Deprivation: Implications for Resilience

Seagrass habitat is strongly constrained by light availability. Decline in benthic light due to anthropogenic activities (e.g. eutrophication, dredging and catchment modification) is a major threat to seagrass ecosystems, both within Australia and internationally. Even in pristine conditions, light available to seagrasses can be highly variable on timescales ranging from seconds to years. This chapter outlines the three primary mechanisms which enable seagrass to adapt to and/or resist temporary light deprivation: (1) consumption of accumulated carbon; (2) reduction in rates of growth and carbon loss; and (3) increased efficiency of radiation capture and usage. The capacity to withstand severe light deprivation ranges from only two weeks for small, colonising seagrass species such as Halophila ovalis , to beyond two years for large, persistent species such as Posidonia sinuosa. This “tolerance time” depends on the magnitude and timing of the light deprivation, current environmental conditions (e.g. temperature and sediment sulphides) as well as preceding conditions. This chapter proposes a simple conceptual model for seagrass resilience to temporary light reduction , combining both resistance (the capacity of seagrass to survive the light deprivation event), and the capacity to recover once the disturbance ends. Data is synthesized for several potential indicators of seagrass resistance to light reduction