Functioning of Philippine seagrass species under deteriorating light conditions

Enhanced eutrophication as well as siltation are important environmental issues in Southeast Asian coastal waters. Both lead to increased turbidity and hence to lower light availability for the originally extensive seagrass beds that fringe most of the Philippine coastlines. Consequent large-scale declines of seagrass beds have been reported from around the world. Since these seagrass meadows have important functions both for local human populations as well as for larger-scale economics, their decline may have unexpected societal consequences. Seagrass beds have wellestablished importance as highly productive carbon-fixing systems, as nursery and foraging grounds for commercial fish and shrimps, and act as sediment trap and breakwater.Longer-term, consistent reduction in light availability to plants may lead to acclimation in surviving species or succession to more shade-adapted communities. Long-term shade effects on tropical seagrass systems have remained unstudied so far, despite the obvious relation to increased eutrophication and siltation. This study looked in detail to the possible responses and capacity to acclimate to substantial and longerterm light deprivation in two common and contrasting Philippine seagrass species: Thalassia hemprichii and Halodule uninervis. The former species is often dominant, comparatively large, long-lived and late successional, whereas the latter is shorterlived, smaller-sized and has a less clear late successional status. A logistically demanding long-term in situ shading experiment was set-up and monitored for one experimental shading year and one subsequent year of recovery at two sites in Northern Mindanao, Philippines (Naawan and Sulawan, both in Misamis Oriental). Shade responses were quantified at several levels: morphometry, shoot dynamics and clonal integration, photosynthetic capacity, carbohydrate and nutrient allocation. Experimental shade levels were approximately 0%, 60% and 85% reduction of full light reaching the seagrass stands. The interaction with increasingly reduced sediment conditions often observed along eutrophication gradients was incorporated in this study.First and foremost, the highest shading level led to pronounced shoot mortality, with little or no new shoot recruitment. T hemprichii survived better than H. uninervis (18% versus 5% of the original shoots remained after one year), probably due to the more robust rhizome network of the former, providing a tighter clonal integration and containing substantial quantities of reserve carbohydrates that could be mobilized. In those few surviving shoots, relative leaf growth rates were maintained across the three shading levels. After removal of the experimental shades, recovery of T hemprichii was faster than that of H. uninervis, but full recovery to pre-experimental densities in the plots that had received the highest shading was estimated to exceed two years in the former and even longer in the latter species.Both species displayed considerable plasticity in shoot morphology as a response to shading: leaf surface area and specific chlorophyll content increased, and also leaf turnover was reduced. No clear response to shading was observed in the photosynthesislight curve parameters. Photosynthetically fixed carbon was substantially less in the shade, and allocation to rhizomes and new shoots was strongly reduced. This is similar to a general plant response to shading: fixed carbon is used for the maintenance of the leaves at the expense of belowground tissue. The longer-lived T hemprichii allocated a comparatively larger proportion to its rhizomes than H. uninervis, in line with its tighter clonal integration. The tracking of allocation of stable isotopes ¹⁵ N and ¹³ C confirmed the difference in clonal integration between the two species. Shade effects were more pronounced when the plants were grown on more reducing sediments: whole-plant diel oxygen and carbon balances remained negative confirming the substantial shoot mortality in these conditions.This dissertation has shown that photosynthetic capacities and relative growth rates are comparatively poor predictors of the long-term effects of light deterioration in our coastal waters. Rather, more conspicuous responses were observed in shoot morphology and shoot dynamics as a consequence of shading, shoot density decreased, leaf length increased, shoot recruitment dropped and chlorophyll content of the leaves of surviving shoots increased. Increased shoot mortality leading to reduced density was due to a substantial reduction in physiological integration, i.e. the translocation of carbohydrates and nutrients among shoots sharing the same horizontal rhizomes was significantly reduced