One of the major features of the coastal zone is that part of its sea floor receives a significant amount of sunlight and can therefore sustain benthic primary production by seagrasses, macroalgae, microphytobenthos and corals. However, the contribution of benthic communities to the primary production of the global coastal ocean is not known, partly because the surface area where benthic primary production can proceed is poorly quantified. Here, we use a new analysis of satellite (SeaWiFS) data collected between 1998 and 2003 to estimate, for the first time at a nearly global scale, the irradiance reaching the bottom of the coastal ocean. The following cumulative functions provide the percentage of the surface (<i>S</i>) of the coastal zone receiving an irradiance greater than <i>E<sub>z</sub></i> (in mol photons m<sup>−2</sup> d<sup>−1</sup>): <br><br> <i>S</i><sub>Non-polar</sub> = 29.61 − 17.92 log<sub>10</sub>(<i>E<sub>z</sub></i>) + 0.72 log<sub>10</sub><sup>2</sup>(<i>E<sub>z</sub></i>) + 0.90 log<sub>10</sub><sup>3</sup>(<i>E<sub>z</sub></i>) <br><br> <i>S</i><sub>Arctic</sub> = 15.99 − 13.56 log<sub>10</sub>(<i>E<sub>z</sub></i>) + 1.49 log<sub>10</sub><sup>2</sup>(<i>E<sub>z</sub></i>) + 0.70 log<sub>10</sub><sup>3</sup>(<i>E<sub>z</sub></i>) <br><br> Data on the constraint of light availability on the major benthic primary producers and net community production are reviewed. Some photosynthetic organisms can grow deeper than the nominal bottom limit of the coastal ocean (200 m). The minimum irradiance required varies from 0.4 to 5.1 mol photons m<sup>−2</sup> d<sup>−1</sup> depending on the group considered. The daily compensation irradiance of benthic communities ranges from 0.24 to 4.4 mol photons m<sup>−2</sup> d<sup>−1</sup>. Data on benthic irradiance and light requirements are combined to estimate the surface area of the coastal ocean where (1) light does not limit the distribution of primary producers and (2) net community production (<i>NCP</i>, the balance between gross primary production and community respiration) is positive. Positive benthic <i>NCP</i> can occur over 33% of the global shelf area. The limitations of this approach, related to the spatial resolution of the satellite data, the parameterization used to convert reflectance data to irradiance, the lack of global information on the benthic nepheloid layer, and the relatively limited biological information available, are discussed.
Submerged vegetation respond to increased nutrient loading through a shift from slow-growing seagrasses and large macroalgae to fast-growing macroalgae, and the ultimate dominance of phytoplankton at high nutrient loadings. This shift reflects a change from nutrient to light limitation along the eutrophication gradient. Slow-growing seagrasses and large macroalgae are good competitors when nutrients are limiting because they have relatively low nutrient requirements, are able of efficient internal nutrient recycling, and can access the elevated nutrient pools in the sediment. Fast-growing macroalgae and phytoplankton are superior competitors when light is limiting because they are positioned closer to the water surface, and capture and use light more efficiently. The important ecosystem consequences of altered nutrient regimes derive from the shift in dominant vegetation types. Slow-growing seagrasses and large macroalgae are longevous, decompose slowly, and experience only moderate grazing losses, whereas the production of fast growing macroalgae and phytoplankton is transferred faster to heterotrophs, through increased grazing and decomposition rates. Recovery of submerged vegetation following nutrient reduction plans is a very slow process, which involves the replacement of fast-growing for slow-growing plants. Simulation models predict recovery times to oscillate between a few years for macroalgae and fast-growing seagrasses to centuries for slow-growing seagrasses following nutrient reduction plans.
The flowering frequency of the Philippine seagrasses Thalassia hemprichii (Ehrenb.) Aschers., Cymodocea rotundata Ehrenb. et Hempr. ex Aschers., and Enhalus acoroides (L.f.) Royle, growing on a reef flat in Bolinao (Pangasinan Province, The Philippines) was examined based on examination of flowering scars on the seagrass shoots. The flowering frequency of C. rotundata and T.: hemprichii was low (0.064 flowers shoot(-1) yr(-1) and 0.125 flowers shoot(-1) yr(-1), respectively), indicating that only a fraction of the shoots of these species will flower during their life spans. Shoots of these species required a maturation period of between half a year and one year before flowering. In contrast, most of the E. acoroides shoots examined had flowered several times, producing, on average, 2.8 flowers shoot(-1) yr(-1). Examination of the past flowering of E. acoroides revealed substantial interannual differences in flowering frequency in the period 1985-1992, with a maximum in 1987. Because of the large flowers of this species, the estimated biomass allocated to flowering was orders of magnitude greater for E. acoroides (35.8 g dw m(-2) yr(-1)) than for C. rotundata (0.021 g dw m(-2) yr(-1)) and T. hemprichii (3.56 g dw m(-2) yr(-1)). These results indicate that sexual reproduction could be a minor sink of resources for C. rotundata and I: hemprichii (<1% of the annual above-ground production), while it may represent a dominant source of losses of resources acquired by E. acoroides (up to 50% of the annual above-ground production). The implications of these contrasting strategies in the flowering effort of the seagrass species examined are, however, unclear, but the large output of sexual propagules of E. acoroides, compared to the other two species, should confer this species a greater capacity to recover after disturbance. [KEYWORDS: Cymodocea-nodosa; growth; dynamics]
Ultraviolet-B (UVB) radiation is a global stressor that has profound impacts on freshwater and marine ecosystems. However, an analysis of the patterns of sensitivity to UVB radiation across aquatic photosynthetic organisms has not yet been published. Here, we performed a meta-analysis on results reported in 214 studies compiled from the published literature to quantify and compare the magnitude of responses of aquatic photosynthetic organisms to changes in UVB radiation. The meta-analysis was conducted on observations of marine (n = 893) and freshwater macroalgae (n = 126) and of marine (n = 1087) and freshwater (n = 2889) microalgae (total n = 4995). Most of these studies (85%) analysed the performance of organisms exposed to natural solar radiation when UVB was partially or totally reduced compared with the organismal performance under the full solar radiation spectrum, whereas the remaining 15% of the studies examined the responses of organisms to elevated UVB radiation mostly using artificial lamps. We found that marine photosynthetic organisms tend to be more sensitive than freshwater photosynthetic organisms to UVB radiation; responses to either decreased or increased UVB radiation vary among taxa; the mortality rate is the most sensitive of the trait responses to elevated UVB radiation, followed by changes in cellular and molecular traits; the sensitivity of microalgae to UVB radiation is dependent on size, with small-celled microalgae more sensitive than large-celled microalgae to UVB radiation. Thick macroalgae morphotypes were the less sensitive to UVB, but this effect could not be separated from phylogenetic differences. The high sensitivity of marine species, particularly the smallest photosynthetic organisms, to increased UVB radiation suggests that the oligotrophic ocean, a habitat comprising 70 % of the world's oceans with high UVB penetration and dominated by picoautotrophs, is extremely vulnerable to changes in UVB radiation.
MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 219:149-158 (2001) - doi:10.3354/meps219149 Effects of seagrass Thalassia testudinum on sediment redox S. Enríquez1, N. Marbà2,*, C. M. Duarte2, B. I. van Tussenbroek1, G. Reyes-Zavala1 1Unidad Académica de Puerto Morelos, Instituto de Ciencias del Mar y Limnología, Universidad Nacional Autónoma de México, Apto. Postal 1152, 77500 Cancún, Quintana Roo, Mexico 2Grup d¹Oceanografia Interdisciplinar, Institut Mediterrani d¹Estudis Avançats (CSIC-UIB), C/Miquel Marquès 21, 07190 Esporles, Illes Balears, Spain *Corresponding author. E-mail: ieanmb@clust.uib.es ABSTRACT: The redox conditions were compared in vegetated versus unvegetated sediments across a range of contrasting Thalassia testudinum (sometimes mixed with Syringodium filiforme) meadows at Puerto Morelos Mexico reef lagoon. Moreover, the role of seagrass photosynthetic activity in affecting the redox conditions was tested in one of the meadows by experimentally reducing seagrass photosynthesis through shading. The seagrass rhizosphere extended 26 to 40 cm into the sediment, and accounted for 23 to 504 g DW m-2 of root material, mostly contributed by T. testudinum. T. testudinum placed 50% (i.e. centroidal depth) and 95% of its root biomass within 12.6 ± 0.58 and 54.4 ± 2.53 cm of the sediment surface, respectively; while S. filiforme placed 50 and 95% of its root biomass within 8.0 ± 0.87 and 34.7 ± 3.8 cm of the sediment surface, respectively. Vegetated sediments presented 50% of positive redox potential anomaly (i.e. redox potential in vegetated sediments - redox potential in adjacent bare sediments), remarkably similar (t-test, p > 0.5) to depths to the centroidal depth of the seagrass roots in the sediments. The shading experiment conducted in situ for 5 d demonstrated that the positive redox anomaly found at depth in vegetated sediments was derived from seagrass photosynthetic activity. The sediments around seagrass rhizosphere in the shaded plots were progressively reduced to reach an average decline of the redox conditions by about 45 mV by Day 5. The results presented show that seagrasses contribute to modify sediment redox conditions around their rhizosphere. KEY WORDS: Thalassia testudinum · Syringodium filiforme · Rhizosphere · Redox conditions · Mexican Caribbean Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 219. Online publication date: September 10, 2001 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2001 Inter-Research.
Abstract Seagrass meadows store globally significant organic carbon (C org ) stocks which, if disturbed, can lead to CO 2 emissions, contributing to climate change. Eutrophication and thermal stress continue to be a major cause of seagrass decline worldwide, but the associated CO 2 emissions remain poorly understood. This study presents comprehensive estimates of seagrass soil C org erosion following eutrophication‐driven seagrass loss in Cockburn Sound (23 km 2 between 1960s and 1990s) and identifies the main drivers. We estimate that shallow seagrass meadows (<5 m depth) had significantly higher C org stocks in 50 cm thick soils (4.5 ± 0.7 kg C org /m 2 ) than previously vegetated counterparts (0.5 ± 0.1 kg C org /m 2 ). In deeper areas (>5 m), however, soil C org stocks in seagrass and bare but previously vegetated areas were not significantly different (2.6 ± 0.3 and 3.0 ± 0.6 kg C org /m 2 , respectively). The soil C org sequestration capacity prevailed in shallow and deep vegetated areas (55 ± 11 and 21 ± 7 g C org m −2 year −1 , respectively), but was lost in bare areas. We identified that seagrass canopy loss alone does not necessarily drive changes in soil C org but, when combined with high hydrodynamic energy, significant erosion occurred. Our estimates point at ~0.20 m/s as the critical shear velocity threshold causing soil C org erosion. We estimate, from field studies and satellite imagery, that soil C org erosion (within the top 50 cm) following seagrass loss likely resulted in cumulative emissions of 0.06–0.14 Tg CO 2‐eq over the last 40 years in Cockburn Sound. We estimated that indirect impacts (i.e. eutrophication, thermal stress and light stress) causing the loss of ~161,150 ha of seagrasses in Australia, likely resulted in the release of 11–21 Tg CO 2 ‐eq since the 1950s, increasing cumulative CO 2 emissions from land‐use change in Australia by 1.1%–2.3% per annum. The patterns described serve as a baseline to estimate potential CO 2 emissions following disturbance of seagrass meadows.
Spartina alterniflora has extensively invaded the coastline of China, including in Maoyan Island of Zhejiang Province. Ecological restoration has been conducted using non-native mangrove Kandelia obovata to replace S. alterniflora in an attempt to restore the impacted intertidal zones. To illustrate the ecological effectiveness of the restoration projects, macrobenthos communities were studied among different habitats within the restored areas, including one non-restored S. alterniflora marsh (SA) and three differently-aged restored K. obovata stands planted in 2003, 2009, and 2011 respectively (KF14, KF8, and KF6). Besides, one unvegetated mudflat (MF) adjacent to the non-restored S. alterniflora marsh and one K. obovata forest transplanted in 2006 (RKF) at a previously barren mudflat without invasion history of S. alterniflora were set as reference sites. A total of 69 species of macrobenthos were collected from Maoyan Island, and the species richness was dominated by gastropoda (23 species), polychaeta (18 species), and malacostraca (16 species). There was no significant difference between the six sites in terms of the abundance of macrobenthos, with the average values of abundance peaking in KF6 (734.7 ind m−2) and being lowest in RKF (341.3 ind m−2). The six sites had significant differences in terms of the biomass of macrobenthos. The KF8 site contained the highest average biomass (168.3 g m−2), whereas the MF site had the lowest (54.3 g m−2). The Shannon-Wiener diversity index and Pielou’s evenness index of the macrobenthos did not exhibit significant differences among the six sites. However, the results of permutational multivariate analysis of variance (PERMANOVA) revealed significant spatial differences in the macrobenthos community structure between the sites. Since KF14 shared a similar macrobenthos community structure with RKF, while representing a strikingly different structure from SA, we infer that ecological restoration using K. obovata can restore the macrobenthos community to resemble to a normally planted K. obovata forest about 15 years after restoration.
