Johan Stapel

Leaf nutrient resorption, leaf lifespan and the retention of nutrients in seagrass systems

Abstract Efficient nutrient resorption from senescing leaves, and extended leaf life spans are important strategies in order to conserve nutrients for plants in general. Despite the fact that seagrasses often grow in oligotrophic waters, these conservation strategies are not strongly developed in seagrasses. A compilation of literature data on nutrient resorption from seagrass leaves shows that the mean resorption of nitrogen is 20.4%, and that of phosphorus 21.9%, which is lower than comparable values for various groups of perennial terrestrial plants. The actual realised resorption in seagrasses may be even less as a result of premature losses of leaf fragments due to herbivory and hydrodynamic stresses, and due to leaching losses. The leaf lifespan in seagrasses on average is 88.4 days, but is highly variable, ranging from 345 days in Posidonia oceanica to only a few days in Halophila ovalis. Leaf lifespan increases with increasing leaf weight, and decreases with increasing leaf formation rate. Furthermore, leaf longevity increases going from tropical to temperate latitudes. We compared seagrass leaf lifespan with those of freshwater angiosperms, terrestrial herbaceous plants, shrubs and trees. Considerable variability in leaf lifespan was also found in these plant groups, but comparison among data sets shows that seagrass leaf lifespan is significantly lower than the leaf lifespan of terrestrial herbaceous plants, shrubs and trees. No significant difference was found between the leaf lifespan of seagrasses and freshwater angiosperms. Leaves are usually the major sink for nutrients in seagrasses. The combination of low nutrient resorption from the leaves and a short leaf lifespan is, therefore, expected to result in a low nutrient residence time in the plants. Indeed, field experiments with 15N labelled Thalassia hemprichii showed that less than 5% of the initial 15N amount was still within the living plant biomass 240 days after labelling. Limited nutrient retention in the plant biomass necessitates the capture of new nutrients for persistent growth. We speculate that effective nutrient uptake by seagrass leaves is an important strategy to maintain an adequate nutrient balance in seagrasses, particularly in thin vegetation or in small patches. The constraints imposed by the marine environment may have favoured the development of this strategy over the development of efficient nutrient conservation strategies.

The limited effect of in situ phosphorus and nitrogen additions to seagrass beds on carbonate and terrigenous sediments in South Sulawesi, Indonesia

Seagrass response to in situ sediment nutrient enrichment with slow-release fertilizers was studied (4 to 5 months after fertilization) in three tropical seagrass beds in South Sulawesi, Indonesia. Nitrogen-addition and P-addition had no significant effects on seagrass biomass, shoot density, and leaf production in mixed seagrass beds in a terrigenous sandy bay and a carbonate sedimentary reef flat environment, simultaneously fertilized in 1990. An additional experiment using a slightly adapted methodology (1991) at another carbonate site, characterized by a more homogeneously vegetated, and largely monospecific seagrass meadow of Thalassia hemprichii, again showed no significant response of seagrass growth, biomass, and shoot density to fertilization. C and N contents of plant tissue, however, had increased significantly (p<0.05) to 12–25% above the controls in response to N-addition at this locality. The relatively high ambient porewater nutrient concentrations (about 10 μM PO4, about 100 μM NH4) and rather low C:N:P ratios (565:18:1) in seagrass tissue, indicate that nutrient supply meets seagrass demand and provide an explanation for the lack of response to the manipulations. The relatively high porewater phosphate concentrations in carbonate sediments are attributed to the relatively coarse particle-size distribution of these sediments in the study area, which limits their adsorption capacity and prevents P-limitation of seagrass growth, which is generally considered to be characteristic of tropical carbonate-rich environments.

The biology of Thalassia: paradigms and recent advances in research

The two species belonging to the genus Thalassia (Family: Hydrocharitaceae), T. testudinum Banks ex Konig and T. hemprichii (Ehrenberg) Ascherson are widely distributed in shallow coastal areas in the tropics and subtropics of the Western Atlantic and Indo-Pacific, respectively (den Hartog, 1970; Phillips and Menez, 1988; Spalding et al., 2003), and they are considered to be ‘twin species’. The current hypothesis, based on paleographical data, is that they

PRIMARY PRODUCTION OF DEEP-WATER HALOPHILA OVALIS MEADOWS

Monospecific meadows of Halophila ovalis (R. Brown) Hooker f. were studied in deep waters (14-16 m) around Langkai island in South Sulawesi, Indonesia, during May-September 1990, The average shoot density of Halophila in biomass samples (n=10) taken from these meadows was 1099 +/- 195 leaf pairs m(-2). Comparison with average leaf pair densities (427 +/- 211 leaf pairs m(-2)) estimated by in situ counts under water (using SCUBA) revealed that more than 50% of the leaf tissue was covered under the sediment. The leaf area index (LAI) was 0.68 +/- 0.32 m(2) m(-2) (including sediment-covered leaves). Total seagrass biomass in the meadows averaged 10.93 +/- 2.65 g ADW m(-2), of which 42% was contributed by above-ground plant parts. Metabolic measurements using m the bell jar-technique and plastochron interval measurements gave comparable results, revealing total plant production to range between 0.83 and 1.38 g C m(-2) day(-1) with a turn-over of total plant biomass of 3.9-6.6 days. Rhizome tagging experiments showed that 34% of this production may be contributed by the below-ground biomass, The light compensation point fur Halophila ovalis at 15 m depth was 33 mu mol photons m(-2) s(-1). The high primary production of these sparsely grown meadows at such considerable depths (14-16m) and under high stress from sedimentation (at the bottom of a reef slope) is attributed to the strongly opportunistic character of this seagrass species, whose meadows can be found to depths of over 30 m in the study area. [KEYWORDS: Halophila ovalis; Indonesia; light limitation; oxygen exchange; production; seagrass; sedimentation South sulawesi; seagrass beds; indonesia; decipiens; biomass; decomposition; queensland; australia; growth]

Abundance, edge effect, and seasonality of fauna in mixed-species seagrass meadows in southwest Sulawesi, Indonesia

Abstract Motile fauna species in two mixed-species seagrass meadows with different canopy structure were studied on an uninhabited island in the Spermonde Archipelago, Sulawesi, Indonesia. The main focus of the study was to assess the edge effect and seasonal abundance of macrobenthic invertebrates. Fish and infauna densities were determined as well. Fauna was counted using permanent transects (macrobenthic invertebrates), visual census (fish species), and sediment cores (infauna). Both meadows had a comparable distribution of motile fauna species with polychaetes (35% of total abundance), bivalves (27%) and sipunculids (25%) accounting for the largest part of the total faunal abundance. The closed canopy meadow (high seagrass leaf biomass) had an overall higher faunal abundance compared with the open canopy meadow (low seagrass leaf biomass) (1133 vs. 751 individuals m−2). Although infauna abundance was comparable between the meadows, macrobenthic invertebrates (crustaceans, echinoderms, and molluscs) and fishes were more abundant in the closed canopy meadow, with only a few individual species more abundant in the open canopy meadow. The effect of distance from the meadow edge on macrobenthic invertebrate abundance was significant, with higher abundances towards the interior of the seagrass meadows, but for fish abundance no significant differences were found. Effects of seasonality (rainy vs. dry season) on macrobenthic invertebrate abundance were only significant for molluscs. We concluded that macrobenthic invertebrate abundance was most influenced by seagrass canopy structure, followed by meadow edge effects, and least by seasonality. Comparisons of faunal abundance in seagrass meadows need thus to include information on these three variables.

Leaf production, shoot demography, and flowering of Thalassodendron ciliatum along the east African coast

Several characteristics of Thalassodendron ciliatum populations were evaluated along the coasts of Kenya and Zanzibar Island, with the aim to study spatial variability in this species. A reconstruction technique, using scars left by abscised leaves and flowers, was employed to determine leaf production, shoot demography, and flowering frequency. Eight subtidal sites in different back-reef lagoons were sampled along with a subtidal site that was not protected by a reef, a site with intertidal rack pools, and a subtidal site in a mangrove bay. Leaf- production rates were lowest for the population of the unprotected subtidal site and for the population from the intertidal rock pools (30 leaves per shoot per year). At these sites, leaf life-span was almost twice as long (94 days). Low leaf-formation rates seem to be compensated by long leaf life- spans to maintain similar numbers of standing leaves per shoot. Highest leaf-production rates were found at the mangrove-bay site (53 leaves per shoot per year). The mangrove-bay population showed internodal lengths of almost 7 mm and stem lengths of almost 90 cm. The stems were two- to four-fold longer than those at the other sites. A short leaf life-span of 51 days at the mangrove site indicates that the investment in stem growth occurs at the expense of leaf maintenance. Median ages of the populations varied almost four-fold from 0.5 to 1.8 years. All meadows showed shoot-recruitment rates that were either the same as or larger than shoot mortality rates, suggesting that the environmental quality in this region is still suitable for sustaining vigorous seagrass vegetation. Flowering frequencies were generally low and seedlings were not found. [KEYWORDS: seagrass; leaf production; demography; flowering; east Africa; Thalassodendron ciliatum Cymodocea-nodosa; reconstructive technique; seagrasses colonization; population-dynamics; den hartog; growth; pollination; amphibolis; biomass; lagoon]