Production and population ecology ofPhyllospadix iwatensis Makino. II. Comparative studies on leaf characteristics,foliage structure and biomass change in an intertidal and subtidal zone

A seagrass in Japan,Phyllospadix iwatensis Makino, is distributed in the lower intertidal zone and upper subtidal zone making a dense population on the Choshi coast, Japan. IntertidalP. iwatensis is able to receive sufficient light for photosynthesis but experienced severe exposure to the air, which decreased a large amount of aboveground biomass in April to June (i.e. the daytime exposure season). SubtidalP. iwatensis was never exposed throughout the year and the aboveground biomass increased gradually over the daytime exposure season. However, the maximum aboveground biomass and shoot density of the subtidal plant never exceeded that of the intertidal plant. The dense foliage, large aboveground biomass and high shoot density of both intertidal and subtidal plants is likely to be an adaptation to heavy water movement, but the subtidal plants always received insufficient light for photosynthesis as a result of having dense foliage, particularly in turbid water. In choppy and swell sea,P. iwatensis did not seem to be adapted to growing in the subtidal zone where there was shortage of light.     

Seagrass–Sediment Feedback: An Exploration Using a Non-recursive Structural Equation Model

The reciprocal effects between sediment texture and seagrass density are assumed to play an important role in the dynamics and stability of intertidal?Ccoastal ecosystems. However, this feedback relationship has been difficult to study empirically on an ecosystem scale, so that knowledge is mainly based on theoretical models and small-scale (experimental) studies. In this paper we apply a non-recursive structural equation model (SEM) to empirically investigate, at large spatial scale, the mutual dependence between seagrass (Zostera noltii) density and sediment texture, on the pristine, seagrass-dominated, intertidal mudflats of the Banc d??Arguin, Mauritania. The non-recursive SEM allows consistent estimation and testing of a direct feedback between sediment and seagrass whilst statistically controlling for the effects of nutrients and abiotic stress. The resulting model is consistent with the hypothesized negative feedback: grain size decreases with seagrass density, whereas fine grain size has a negative impact on seagrass density because it decreases pore water exchange which leads to hypoxic sediment conditions. Another finding is that seagrass density increases with sediment organic material content up to a threshold level beyond which it levels off. In combination with decreasing grain size, accumulation of organic matter creates hypoxic sediment conditions which lead to the production of toxic hydrogen sulfide which slows down seagrass growth. The negative feedback loop implies that intertidal Z. noltii modifies its own environment, thus controlling its growing conditions. To the best of our knowledge, this study is the first to demonstrate a direct negative feedback relationship in ecosystems by means of a non-recursive SEM.     

Material exchange and food web of seagrass beds in the Sylt-Rømø Bight: how significant are community changes at the ecosystem level?

Material exchange, biodiversity and trophic transfer within the food web were investigated in two different types of intertidal seagrass beds: a sheltered, dense Zostera marina bed and a more exposed, sparse Z. noltii bed, in the Northern Wadden Sea. Both types of Zostera beds show a seasonal development of above-ground biomass, and therefore measurements were carried out during the vegetation period in summer. The exchange of particles and nutrients between seagrass beds and the overlying water was measured directly using an in situ flume. Particle sedimentation [carbon (C), nitrogen (N) and phosphorus (P) constituents] from the water column prevailed in dense seagrass beds. In the sheltered, dense seagrass bed, a net particle uptake was found even on windy days (7–8 Beaufort). Dissolved inorganic N and orthophosphate were mainly taken up by the dense seagrass bed. At times of strong winds, nutrients were released from the benthic community to tidal waters. In a budget calculation of total N and total P, the dense seagrass beds were characterised as a material sink. The seagrass beds with sparse Z. noltii were a source of particles even during calm weather. The uptake of dissolved inorganic N in the sparse seagrass bed was low but significant, while the uptake of inorganic phosphate and silicate by seagrasses and their epiphytes was exceeded by release processes from the sediment into the overlying water. Estimates at the ecosystem level showed that material fluxes of seagrass beds in the Sylt-Rømø Bight are dominated by the dense type of Zostera beds. Therefore, seagrass beds act as a sink for particles and for dissolved inorganic nutrients. During storms, seagrass beds are distinct sources for inorganic nutrients. The total intertidal area of the Sylt-Rømø Bight could be described as a sink for particles and a source for dissolved nutrients. This balance of the material budget was estimated by either including or excluding seagrass beds. Including the subtidal part, the function of the ecosystem as a source for particles increased, supposing that all seagrass beds were lost from the area. During the vegetation period, seagrass beds act as a storage compartment for material accumulated in the living biomass of the community. There was great biodiversity among the plant and animal groups found in intertidal seagrass beds of the Sylt-Rømø Bay, representing 50–86% of the total number of species investigated, depending on the particular group. Since most species are not exclusively seagrass residents, the loss of intertidal seagrass beds would be of minor importance for biodiversity at the ecosystem level. Food web structure in seagrass beds is different from other intertidal communities. Primary production and detritus input is high, but secondary production is similar to that of unvegetated areas, although the relative importance of the trophic guilds is different. The loss of seagrass beds leads to profound alterations in the food web of the total ecosystem. Historical as well as recent changes in material fluxes and energy flow due to man-made alterations to the ecosystem are discussed.     

The effects of El Niño‐Southern Oscillation events on intertidal seagrass beds over a long‐term timescale

El Niño‐Southern Oscillation (ENSO) events can cause dramatic changes in marine communities. However, we know little as to how ENSO events affect tropical seagrass beds over decadal timescales. Therefore, a diverse array of seagrass (Thalassia hemprichii) habitat types were surveyed once every 3 months for 16 years (January 2001 to February 2017) in a tropical intertidal zone that is regularly affected by both ENSO events and anthropogenic nutrient enrichment. La Niña and El Niño events had distinct effects on the biomass and growth of T. hemprichii. During La Niña years, higher (a) precipitation levels and (b) seawater nitrogen concentrations led to increases in seagrass leaf productivity, canopy height, and biomass. However, the latter simultaneously stimulated the growth of periphyton on seagrass leaves; this led to decreases in seagrass cover and shoot density. More frequent La Niña events could, then, eventually lead to either a decline in intertidal seagrass beds or a shift to another, less drought‐resistant seagrass species in those regions already characterized by eutrophication due to local anthropogenic activity.     

Habitat linkages: the effect of intertidal saltmarshes and adjacent subtidal habitats on abundance, movement, and growth of an estuarine fish

In this study we used pinfish (Lagodon rhomboides) in field experiments to examine linkages between intertidal saltmarsh and adjacent subtidal habitats. Pinfish are more than twice as abundant in intertidal marshes adjacent to seagrass beds than in those adjacent to the unvegetated subtidal bottom. Movement of pinfish between the marsh edge and the adjacent subtidal habitat was greater for fish captured in areas with both intertidal and subtidal vegetation than in those with intertidal vegetation and adjacent unvegetated mudflats. This movement provides an important link between habitats, allowing transfer of marsh-derived secondary production to subtidal seagrass beds and vice versa. Pinfish held in enclosures with both intertidal and subtidal vegetation were, on average, approximately 90% heavier than fish held in enclosures with intertidal vegetation and unvegetated subtidal bottom. Because saltmarshes and seagrass beds contribute to the production of living marine resources, active measures are being taken to preserve and restore these habitats. The results from this study have direct application to decisions concerning site selection and optimal spatial proximity of saltmarsh and seagrass habitats in the planning of restoration and mitigation projects. To maximize secondary production and utilization of intertidal marshes, managers may opt to restore and/or preserve marshes adjacent to subtidal seagrass beds. Received: 31 May 1996 / Accepted: 23 October 1996     

The intertidal mudflats of Barr Al Hikman,Sultanate of Oman,as feeding,reproduction and nursery grounds for brachyuran crabs

Brachyuran crabs are an important ecological and economical, yet often unstudied aspect of intertidal mudflats of the Arabian Peninsula. Here we provide baseline density estimates of crabs at the relatively pristine intertidal mudflats of Barr Al Hikman (Sultanate of Oman) and provide information on their life cycle and habitat preference. Across the winters of 2012–2015 crabs were sampled on a grid covering the entire intertidal depth gradient. 29 species were found and average densities varied between 12 and 54 crabs/m². Deposit-feeding and herbivorous crabs were the most abundant species across all winters. Size frequency data and the presence of ovigerous females show that most crabs species reproduce in the intertidal area. P. segnis, the most important crab for local fisheries, was found to use the intertidal area as a nursery ground. We analysed the relationships between the two most abundant crab species, Macrophthalmus sulcatus and Thalamita poissonii and the environmental variables: seagrass density, tidal elevation, median grain size and sediment depth using Random Forest models. The predictive capacity of the models and the relative importance of the environmental predictors varied between years, but crab densities in general were positively associated with seagrass density, presumably because seagrass offers feeding habitat.

     

Differential population response of allocation,phenology, and tissue chemistry in <Emphasis Type="Italic">Spartina alterniflora</Emphasis>

Phenotypic variation within species is widespread among salt marsh plants. For Spartina alterniflora, the dominant species of low intertidal wetlands across the Altantic and Gulf coasts of the US, distinct phenological and morphological differences among populations from different latitudes have been found. To determine whether S. alterniflora plants from lower latitudes and those regenerated from Delaware tissue cultures would maintain differences from that of native plants, we conducted a field study in a natural salt marsh in Delaware, US. After two growing seasons, plant height, stem density, above- and belowground biomass, elemental composition, and nutrient resorption were measured. Natural variation in porewater salinity influenced physiological traits of Na⁺/K⁺ ratio regulation and nitrogen resorption efficiency similarly across populations. While plant height exhibited plasticity where populations tended to converge to a similar height, several other traits remained distinct. Delaware plants had a greater rate of rhizome growth than Georgia and Louisiana plants, which correlated with a greater magnitude of fall senescence. If traits such as seasonal translocation are plastic and can change with the length of the growing season, climate warming may alter belowground biomass production of S. alterniflora in wetlands of the mid-Atlantic.     

Seasonal changes in environmental variables,biomass, production and nutrient contents in two contrasting tropical intertidal seagrass beds in South Sulawesi,Indonesia

Seasonal dynamics were studied by monthly monitoring of biological and environmental variables in permanent quadrats in two contrasting intertidal seagrass beds in South Sulawesi, Indonesia, from February 1991 to January 1992. Datasets were analysed with canonical correlation analysis for correlations between environmental and biological variables. Considerable variation in biomass, production and plant tissue nutrient contents in a monospecific seagrass bed of Enhalus acoroides, growing on a coastal terrigenous mudbank (Gusung Tallang), was assumed to be related to riverine influences of the nearby Tallo River. The variation in seagrass variables at this site could, however, not be significantly correlated to seasonal patterns in rainfall, salinity, tides, nutrient availability, water motion or turbidity. A seasonal cycle in biomass, production and nutrient contents in a mixed seagrass bed of Thalassia hemprichii and E. acoroides, growing on carbonate sand on the reef flat of an offshore coral island (Barang Lompo), was found to be largely determined by tidal exposure and water motion. Exposure of the intertidal seagrass bed during hours of low water during spring tides showed a gradual shift from exposure during the night (January-June) to exposure during daylight (July-December). Daylight exposure resulted in a significant loss of above-ground plant biomass through desiccation and burning of leaves. The observed seasonal dynamics of the seagrass bed on reef sediment contrast with reports from the Caribbean, where the effect of tidal exposure on comparable shallow-water seagrass communities is relatively insignificant due to a small tidal amplitude.     

Effects of an epiphytic alga on abundances of fish and decapods associated with the seagrass Zostera capricorni

During a field experiment to test effects of changes to leaf height and density of Zostera capricorni on associated fish and decapods, more than one-third of the substratum at some sites was covered by the epiphytic brown alga Giffordia mitchelliae. This paper reports apparent effects of the alga on the 13 most abundant species of fish and decapods, and interactions between effects of the alga and those due to height and density of the seagrass leaves. Abundances of three species were significantly lower, and those of two species were significantly greater where the alga occurred. There were interactions between effects of the alga and those of leaf height and/or density for another four species. In all four cases, when the alga had an effect, it was to decrease abundance. The effects of Giffordia were generally opposite to those reported for drift algae in seagrass habitats elsewhere. The lower abundances of seven species where algal cover was great are best explained by an existing model for seagrass habitats that predicts reductions in abundance of individuals when the ratio of plant material to substratum is very high.     

Spatial and temporal variation of photosynthesis in intertidal Mazzaella laminarioides (Bory)Fredericq (Rhodophyta, Gigartinales)

The red alga Mazzaella laminarioides is an economically important species with an extended latitudinal distribution along the Chilean coast. Its populations form mid-intertidal stands, several meters wide, and therefore are differentially exposed to environmental variables that result in temporal and spatial variability in productivity. We evaluated the effect of latitude and intertidal height on productivity by in situ measurement of photosynthetic performance. Daily and seasonal variations of O₂-evolution rate and maximal quantum yield (F _v/F _m) were determined in plants from the upper and lower intertidal zone at two localities 1500,km apart. Results suggest that plant responses were mainly affected by irradiation, temperature and desiccation. At local level, upper intertidal plants showed a reduced photosynthetic rate and quantum efficiency as compared to those displayed by plants from the lower intertidal, indicating their higher level of excitation energy acclimation. Stronger acclimation differences between upper and lower intertidal plants were observed in spring and summer. Differences in photosynthetic parameters between reproductive phases were recorded in autumn and winter, regardless of the position of the individuals in the intertidal zone. The effects of tidal elevation on seasonal patterns of photosynthesis were also influenced by latitude. Seasonal variation in photosynthetic efficiency was observed in plants from the northern population at both intertidal elevations, but only at the upper intertidal level in the southern population. This study shows that production variability in M. laminarioides results from differences in the intensity of environmental factors observed seasonally at local (intertidal) and latitudinal scales.     

The distribution of seagrass (Zostera noltii) in the Ria Formosa lagoon system and the implications of clam farming on its conservation

Ria Formosa lagoon in southern Portugal has an important population of seagrasses that includes Zostera noltii in the intertidal area. The area is classified as a Natural Park and supports a major economic activity – clam farming. This activity has a direct influence on Z. noltii populations by removal of seagrass beds and altering habitat. Geographic Information Systems and spatial analysis were used to produce the first distribution map of Z. noltii in the Ria Formosa and to analyse interactions between clam farming and the level of seagrass protection according to the Natural Park of Ria Formosa. Stakeholder analyses, using interviews, questionnaires and participant observation, were conducted to gain a better understanding of clam farming's influence on seagrass populations. Seagrass covers 45% of the intertidal area while clam farming covers 14%. An additional 75% of the total area of Z. noltii can be potentially converted into clam cultures. The current management of clam farming is largely ineffective, representing a significant obstacle to seagrass conservation.     

The interspecific biomass–density relationship for terrestrial plants: where do clonal red seaweeds stand and why?

For crowded stands of terrestrial plants, ranging from mosses to trees, plant (or ramet, for clonal plants) density is negatively related to stand biomass. Stand biomass and ramet density were determined for Mazzaella cornucopiae and for Pterocladiella capillacea , two morphologically distinct intertidal clonal red seaweeds, to compare them with terrestrial plants. For these seaweeds, ramet densities were similar to the highest values reported for terrestrial plants (mosses, specifically). Stand biomass was higher than average values expected from the terrestrial interspecific biomass–density relationship, but lower than the limits expected from the terrestrial ultimate biomass–density line. These seaweeds show unexpectedly low ramet slenderness and high biomass packing per unit of volume, compared with the trend observed for terrestrial plants. Possible explanations for these differences are related to the particular physiology and habitat of intertidal clonal seaweeds.     

Dugong habitat use in relation to seagrass nutrients,tides, and diel cycles

The dugong is the only herbivorous mammal that is strictly marine and a seagrass community specialist. The pasture available to the dugong varies with the tides because seagrass occurs in both intertidal and subtidal areas. We GPS-tracked seven dugongs within a 24 km², intensively used seagrass habitat in subtropical Australia in winter. We modeled resource selection within the habitat by comparing the dugongs’ use of space with the distribution of seagrass in an area defined using the combined space-use of the tracked animals. Selection by dugongs for seagrass quantity (biomass) and quality (nutrients) was analyzed within six time/tide combinations to examine the influences of tidal periodicity and the diel cycle on resource selection. Dugong habitat use was consistently centered over seagrass patches with high nitrogen concentrations, except during the day at low tides when the animals had fewer habitat choices and their space use was centered over high seagrass biomass. The association of dugongs with seagrass high in starch was positive during both day and night high tides when the animals could access the intertidal areas where seagrass biomass was generally low. Associations between dugongs and seagrass species were less definite, reflecting the potential for dugongs to exploit several species. Our model of dugong resource selection suggests that nitrogen is the primary limiting nutrient for dugong populations and also confirms the preference of dugongs for high-energy foods.     

Growth and establishment of mangrove (Avicennia marina) on the coastlines of Kuwait

Mangrove (Avicennia marina) communities constitute an importantcomponent of the renewable natural vegetation resources of the coastalzones of the Arabian Gulf south of Kuwait. Attempts to introduce themangrove into the intertidal zones of the coastline of Kuwait are underwaybecause of the expected favorable environmental impact of this plant, andbecause of its moderating influence on the local climate and enhancementof the aesthetic and recreational value of the coastlines and fisheryresources. Propagules were collected from natural stands of mangrove inBahrain and the United Arab Emirates. Silvicultural practices were followedto produce healthy plants after growing of propagules in the greenhouse.Observations indicated that there is a positive correlation betweenpropagule weight and the length of the seedling stem. Outplanting wasdone in two sites in the intertidal zone around the Sulabikhat Bay, Kuwait.Data on plant survival showed that the most suitable level for successfulestablishment of mangrove was around or below the 3-meter contour tidalline. After 7 years of growth, plants reached a mean height of 250 cm,which is close to the height of their parent trees in Bahrain. The U.A.E.stock plants reached 288 cm after 5 years of growth in Kuwait. Plantsflowered and produced viable seeds. The biological feasibility ofestablishing mangrove plantations in the intertidal flats of Kuwait's coastlineswas documented from the field observation of the experimental plantations.Studies to document the environmental impact of the introduction areunderway.     

UV-B-induced plant stress as a possible cause of ten-year hare cycles

Predation has been assumed to be a necessary factor in the ten-year population cycle of the snowshoe hare (Lepus americanus) and Canadian lynx (Lynx canadensis). The UV-B-induced plant stress hypothesis, in contrast, predicts that hare performance, especially reproduction, is negatively related to sunspot numbers, because production of UV-B-protective phenolics in food plants in periods of low sunspot activity, when the ozone layer is thin, increases the availability of amino acids and reduces the amount of phenolics that protect against herbivores. In accordance with the UV-B-induced plant stress hypothesis, and despite the absence of predators that have been assumed to be necessary for hare cycles, mountain hare (Lepus timidus) populations in Norway fluctuate in close synchrony with snowshoe hare populations in Alberta and the Yukon, Canada. When adjusting for the phase of the hare cycle, the natality of snowshoe hare in Alberta 1962–1976 was negatively related to sunspot numbers with a time lag of two years. It is concluded that delayed responses to UV-B-induced changes in plant chemistry during the sunspot cycle is a possible cause of ten-year cycles of hares and other herbivores, for example grouse and forest moths.     

Interactive effects of shade and surface orientation on the recruitment of spirorbid polychaetes

A series of observations and an experiment were carried out to test hypotheses about the effects of shade on the densities of spirorbid polychaetes (Neodexiospira spp.) on intertidal pneumatophores (mangrove roots) of Avicennia marina. Densities of spirorbids were greater on pneumatophores surrounded by seagrass (Zostera mucronata) than patches without seagrass. Within patches of seagrass, the density and survivorship of spirorbids on pneumatophores was greater near the substratum (covered by seagrass) than high above the substratum (not covered by seagrass). The model that these patterns of abundance are explained by greater recruitment of spirorbids to shaded surfaces was assessed. This was done by experimentally testing the hypothesis that recruitment to patches without seagrass would not differ between the upper (unshaded) and lower surfaces (unshaded) of clear plastic sheets, but would be greater on the lower surfaces (shaded) than upper surfaces (unshaded) of black plastic sheets. Recruitment was consistent with these predictions and therefore provided evidence that differences in densities of spirorbids between substrata with and without seagrass may be due largely to differences in shading.     

Spatial and temporal variations in nematode assemblages in tropical seagrass sediments

The changes in the community dynamics of infaunal nematodes associated with tropical Australian intertidal seagrasses at 4 estuarine sites were investigated through 3 seasons (autumn, winter and spring). Nematode densities were highest in winter in all but one of the sites, ranging from 1971 to 3084 inds./10 cm², with one site showing a highest density of 3411 inds./10 cm² in spring. Multiple regression revealed significant correlations between nematode density peaks and seasonal changes in temperature, salinity and surface seagrass cover. Non-metric Multi-Dimensional Scaling revealed that the communities were characterised by relatively low within-site spatial variability but relatively high temporal variability through the three seasons. This temporal variability was largely due to significant increases in abundance of epistrate-feeding species in winter and spring. An investigation of the dominant epistrate-feeding nematodes revealed that predominantly infaunal species were responsible for overall winter and spring density increases. This study provides further evidence of the role played by temperature in regulating tropical, intertidal meiobenthic communities but also indirectly provides evidence of micro-scale seagrass canopy effects (micro-algal supply and availability) that may further enhance the impacts of these larger-scale seasonal environmental changes on the infaunal nematode community.     

Primary production of a seagrass bed on Kavaratti Atoll (Laccadives)

Summary Along the intertidal zone of Kavaratti Lagoon, a seagrass bed, consisting largely of Thalassia and Cymodocea, grows on a substratum of loose coral sand and coral debris. Primary production of this grass bed was estimated from a study of diurnal changes in dissolved oxygen over the bed and experiments were conducted on some isolated plants to determine their rates of oxygen production and consumption. Both these studies showed that photosynthesis of the community during daylight is greater than its respiration during day and night combined.     

Effects of competition,herbivory and substrate disturbance on growth and size structure in pin cherry (Prunus pensylvanica L.) seedlings

We examined separate and interactive effects of intraspecific competition, vertebrate browsing and substrate disturbance on the growth and size structure of pin cherry (Prunus pensylvanica L.) in the first two seasons of growth after clearcutting, in a hardwoods forest in New Hampshire, United States. Over the 15-month study period, 97.5% of 1801 individuals survived, and mean plant height increased from 4-fold at high density to 5-fold at low density. Relative height growth was significantly lower at higher plant densities in two of the three growth periods examined. Vertebrate browsers (moose and deer) significantly preferred taller plants. Browsed plants had higher relative height growth following browsing than unbrowsed plants (compensatory growth) at low and intermediate densities. The degree of compensation declined with density and compensation was not significant at the highest density level. At low and intermediate densities, plants browsed early in life regained height dominance through compensatory growth; they failed to regain dominance at high density. Because compensatory growth tended to offset the effects of size-selective browsing, there was no difference in the degree of size inequality between browsed and unbrowsed plots. However, size inequality increased with plant density. Substrate disturbance caused by logging had no significant effects on either relative height growth or size inequality. The slope of the relationship between relative height growth and initial height increased significantly with density and time, and was higher in unbrowsed than in browsed plots, suggesting that competition among plants was size-asymmetric. Despite the preference of browsers for large plants, there was a clear net growth advantage for plants of large initial size, when the effects of competition, browsing and compensatory growth were combined. The interactive effects of density and browsing demonstrate the importance of a multifactorial approach to the analysis of individual plant performance and population structure.     

Solar activity affects avian timing of reproduction

Avian timing of reproduction is strongly affected by ambient temperature. Here we show that there is an additional effect of sunspots on laying date, from five long-term population studies of great and blue tits (Parus major and Cyanistes caeruleus), demonstrating for the first time that solar activity not only has an effect on population numbers but that it also affects the timing of animal behaviour. This effect is statistically independent of ambient temperature. In years with few sunspots, birds initiate laying late while they are often early in years with many sunspots. The sunspot effect may be owing to a crucial difference between the method of temperature measurements by meteorological stations (in the shade) and the temperatures experienced by the birds. A better understanding of the impact of all the thermal components of weather on the phenology of ecosystems is essential when predicting their responses to climate change.