Are vascular epiphytes nitrogen or phosphorus limited? A study of plant (15) N fractionation and foliar N : P stoichiometry with the tank bromeliad Vriesea sanguinolenta

Although there is unambiguous evidence for vascular epiphytic plants to be limited by insufficient water and nutrient supply under natural conditions, it is an open debate whether they are primarily phosphorus (P) or nitrogen (N) limited. Plant (15) N fractionation and foliar N : P stoichiometry of a tank epiphyte (Vriesea sanguinolenta), and its response to combined N-P fertilization, were studied under semi-natural conditions over 334 d to clarify the type of nutrient limitation. Plants collected in the field and experimental plants with limited nutrient supply showed significant plant (15) N fractionation (mean 5‰) and plant N : P ratios of c. 13.5. Higher relative growth rates and declines in plant (15) N fractionation (0.5‰) and in foliar N : P ratios to 8.5 in the high N-P treatment indicated that these epiphytes were P limited in situ. The critical foliar N : P ratio was 10.4, as derived from the breakpoint in the relationship between plant (15) N fractionation and foliar N : P. We interpret the widespread (15) N depletion of vascular epiphytes relative to their host trees as deriving from (15) N fractionation of epiphytes as a result of P limitation. High foliar N : P ratios (> 12) corroborate widespread P limitation (or co-limitation by N and P) of epiphytic bromeliads and, possibly, other epiphyte species.     

Detection of inconspicuous epiphytic algae supporting food webs in seagrass meadows

Summary Detritus from common seagrasses and other marine angiosperms may often be a less important basis for estuarine food webs than previously believed. In NW Gulf of Mexico seagrass meadows, epiphytic algae have high productivities, palatability, and a more important trophic role than common large plants have. Interdisciplinary field experiments show (1) intensive night-time ingestion of epiphytes by various invertebrate detritivores, (2) very high productivity of epiphytic algae on seagrasses, and (3) assimilation of epiphytes rather than seagrasses, as measured by ¹³C comparisons. These combined data show that many naturally concentrated and potentially competing invertebrates in Gulf of Mexico seagrass meadows feed largely on the algal overgrowth on seagrass blades, even when such algae appear to be sparse. Primary productivity of these epiphytic algae can equal that of the seagrasses, per blade or per unit biomass. Animal ¹³C values tracked epiphytic values rather than seagrass values when comparisons were made over six sites. These measurements reinforce the view that epiphytic algae can be the primary basis of the food web in seagrass meadows.Contribution No. 608 of The University of Texas Marine Science Institute     

Variable responses within epiphytic and benthic microalgal communities to nutrient enrichment

We evaluated how changes in nutrient supply altered the composition of epiphytic and benthic microalgal communities in a Thalassia testudinum (turtle grass) bed in Florida Bay. We established study plots at four sites in the bay and added nitrogen (N) and phosphorus (P) to the sediments in a factorial design. After 18, 24, and 30 months of fertilization we measured the pigment concentrations in the epiphytic and benthic microalgal assemblages using high performance liquid chromatography. Overall, the epiphytic assemblage was P-limited in the eastern portion of the bay, but each phototrophic group displayed unique spatial and temporal responses to N and P addition. Epiphytic chlorophyll a, an indicator of total microalgal load, and epiphytic fucoxanthin, an indicator of diatoms, increased in response to P addition at one eastern bay site, decreased at another eastern bay site, and were not affected by P or N addition at two western bay sites. Epiphytic zeaxanthin, an indicator of the cyanobacterial/coralline red algae complex, and epiphytic chlorophyll b, an indicator of green algae, generally increased in response to P addition at both eastern bay sites but did not respond to P or N addition in the western bay. Benthic chlorophyll a, chlorophyll b, fucoxanthin, and zeaxanthin showed complex responses to N and P addition in the eastern bay, suggesting that the benthic assemblage is limited by both N and P. Benthic assemblages in the western bay were variable over time and displayed few responses to N or P addition. The contrasting nutrient limitation patterns between the epiphytic and benthic communities in the eastern bay suggest that altering nutrient input to the bay, as might occur during Everglades restoration, can shift microalgal community structure, which may subsequently alter food web support for upper trophic levels.     

FIELD MEASUREMENTS OF INORGANIC NITROGEN UPTAKE BY EPIFLORA COMPONENTS OF THE SEAGRASS POSIDONIA OCEANICA (MONOCOTYLEDONS,POSIDONIACEAE)1

Crustose corallines, crustose and erect brown algae, and sessile animals are major components of the epiphytic community of the Mediterranean seagrass Posidonia oceanica (L.) Delile. Production, biomass, and specific composition of this epiphyte–seagrass association are impacted by anthropogenic increase of nutrient load in this oligotrophic area. In this context, nitrogen uptake by P. oceanica and its epiflora was measured using the isotope ¹⁵N at a 10 m depth in the Revellata Bay (Corsica, Mediterranean Sea). Epiflora components showed various seasonal patterns of biomass and abundance. The epiphytic brown algae appeared at the end of spring, later than the crustose corallines, and after the nitrate peak in the bay. Because of their later development in the season, epiphytic brown algae mostly rely on ammonium for their N needs. We hypothesize that the temporal succession of epiphytic organisms plays a crucial role in the N dynamics of this community under natural conditions. The epiphytic brown algae, which have a growth rate one order of magnitude greater than that of crustose corallines, showed lower N‐uptake rates. The greater N‐uptake rates of crustose corallines probably reflect the greater N requirements (i.e., lower C/N ratios) of red algae. We determined that the epiflora incorporated ammonium and nitrate more rapidly than their host. Nevertheless, when biomass was taken into account, P. oceanica was the most important contributor to N uptake from the water column by benthic macrophytes in this seagrass bed.     

Plant–animal associations in two species of seagrasses in Western Australia

Relationships between algal epiphytes and epifaunal invertebrates (amphipods, molluscs and polychaetes) occurring within meadows of the seagrasses Posidonia sinuosa and Amphibolis griffithii were compared along the south west coast of Western Australia. Although the seagrasses are very different structurally, many species of algal epiphytes and epifaunal grazers were common to both. However, meadows of Amphibolis supported a greater number of both algal epiphyte and epifaunal species. The long-lived stems of Amphibolis supported a larger biomass of algal epiphytes and grazers than did the leaves of either Posidonia or Amphibolis. The densities and biomass of epifauna were variable but on a comparison adjusted to the biomass of seagrass, both the density and biomass of the taxonomic groups were similar between seagrass species except that the density of grazing gastropods and the biomass of polychaetes were greater in Amphibolis (by 238% and 252%, respectively). Nested analyses of variance (ANOVA) indicated that variations in plant and animal biomass differed at all spatial scales (sites, meadows within sites and replicates) and the pattern was inconsistent amongst biota. However, a significant proportion of the variability occurred between replicate samples. Canonical correlation and multiple regression analyses indicated that associations between algal epiphytes and epifauna were also inconsistent and differed between seagrass species. These patterns highlight the importance of seagrass species and structural complexity in affecting both the epiphytic and grazer community. The importance of spatial scales at which seagrasses and their associated communities are sampled are equally important because of the differing levels of spatial patchiness.     

Differential anaerobic decomposition of seagrass (Zostera noltii) and macroalgal (Monostroma obscurum) biomass from Arcachon Bay (France)

Arcachon Bay is characterized by extensive meadows of the seagrass Zostera noltii. Moreover, as a consequence of eutrophication, massive proliferations of the macroalga (Monostroma obscurum) have occurred since the beginning of 1990s.This paper describes the anaerobic decomposition of biomass of both species under experimental conditions by two methods. Firstly, the dynamics of decomposition were studied in situ using litter bags. The remaining biomass and the elemental composition of the decomposing macrophytes were monitored. Secondly, degradation was studied in experimental containers under anoxic conditions in which the release of inorganic nutrients and the development of fermentative and sulfate-reducing bacterial populations were followed.The decomposition rate of total biomass was faster for macroalgae than for the vascular plants, thus corroborating previous observations. However, both in situ and laboratory experiments showed that the anaerobic decomposition of the seagrass Z. noltii resulted in rapid release of inorganic N and P, and increasing C/N and C/P ratios of the residual biomass. As a result, the recycling of inorganic nitrogen and phosphorus compounds was slightly more efficient for Z. noltii than for M. obscurum. Recycling of inorganic nutrients appears to be of a great importance to the whole ecosystem, because of the extensive spreading of Z. noltii in the bay.     

Effect of nutrient availability, grazer assemblage and seagrass source population on the interaction between Thalassia testudinum (turtle grass) and its algal epiphytes

Seagrass leaves are often densely covered by epiphytic algae which can suppress seagrass productivity and has been implicated in declines of seagrass meadows worldwide. The net effect of epiphytes on seagrass growth and morphology depends on the independent and interactive effects of a variety of factors, including nutrient availability and the intensity of grazing on epiphytes. Here I report the results of a mesocosm experiment designed to test the effects of nutrient addition and within-functional group variation (grazer species composition and the source population of seagrass) on the strength of the interactions among grazers, epiphytes, and turtle grass (Thalassia testudinum). Turtle grass ramets from two sites in the northern Gulf of Mexico were cleared of epiphytes and transplanted into common-garden mesocosms. Replicate ramets were grown in a split-split plot design with two levels of dissolved nutrients and four different grazer species combinations (Tozeuma carolinense alone, Pagurus maclaughlinae alone, both species together, and no grazers present). As expected, grazers had a significant negative effect on epiphyte biomass/leaf area and a significant positive effect on turtle grass growth in the mesocosms. The two species were more similar in their direct effects on epiphyte biomass than in their indirect effects on turtle grass growth; this may reflect differences in epiphyte community composition under different grazer treatments. The effect of nutrient addition on turtle grass growth depended critically on the intensity of grazing: in the presence of grazers, turtle grass tended to produce a greater biomass of new leaf tissue in the tanks with nutrients added than in the control tanks. However, when grazers were absent, the direction of the effect was reversed, and plants with nutrients added grew less than the control plants. The two source populations of turtle grass differed significantly in epiphyte biomass/leaf area accrued in the mesocosms as well as in the strength of the effect of grazers on turtle grass growth. This suggests that population differentiation in seagrass interactions with epiphytes, as well as spatial and temporal variation in resources and grazer community composition, can greatly effect the role of epiphytes in limiting seagrass productivity.     

Space competition between seagrass and Caulerpa prolifera (Forsskaal) Lamouroux following simulated disturbances in Lassing Park, FL

An experimental study was conducted in Tampa Bay, FL to examine the response to disturbance of two co-occurring subtidal plants: the alga Caulerpa prolifera and the seagrass Halodule wrightii (Ascherson). Some recent studies have called into question the assumption that fast-growing rhizoidal Caulerpa species have the potential to outcompete and rapidly replace local seagrasses. In the Fall of 2002 an abrupt appearance of Caulerpa prolifera was noted in a shallow embayment in Tampa Bay previously dominated by seagrasses. Natural disturbance events were simulated by excavating 0.5 × 0.5 m plots in an area with monospecific C. prolifera and mixed C. prolifera and H. wrightii. Above and below-ground biomass were removed, and recovery of above-ground cover into the newly created gaps was monitored over 15 months. In addition to measuring the recovery of both species, the spatial pattern of Caulerpa recovery from the simulated disturbances was also analyzed. Simulated gaps were rapidly (5-8 months, depending on sampling resolution) and exclusively reoccupied by C. prolifera, with the recovery occurring predominantly via lateral expansion from gap edges rather than colonization by fragments. Therefore, while rhizoidal algae may or may not be able to supplant existing seagrasses by overgrowth or other forms of direct competition, disturbance events that remove seagrass and create bare areas may allow C. prolifera to replace seagrasses over time via preemption of space should an algal bloom such as this be persistent.     

Qualitative and quantitative aspects of the epiphytic component in a mixed seagrass meadow from Papua New Guinea

During 1982, structural and functional aspects of the epiphytic component in a tropical mixed seagrass meadow, have been investigated for each seagrass species separately. This meadow consisted of the seagrasses Thalassia hemprichii (Ehrenb.) Aschers., Cymodocea serrulata (R.Br.) Aschers. et Magnus, C. rotundata Ehrenb. et Hempr. ex Aschers., Halodule uninervis (Forssk.) Aschers. and Syringodium isoetifolium (Aschers.) Dandy.No significant differences were observed in floristic composition, number of algal species, abundance and diversity of the epiphytic component. On an area basis, annual mean above-ground biomass (seagrass leaves and epiphytes), amounted to 82 g ADW, of which 18% could be ascribed to the epiphytic component. The contribution of the epiphytic component to the annual mean above-ground production ranged from 16% on leaves of Thalassia hemprichii to 33% on leaves of Cymodocea serrulata. Total annual mean epiphyte production was 4.6 g ADW m⁻² sediment surface day⁻¹ (19%).When including the macroalgal component of this mixed seagrass meadow, total annual mean above-ground plant biomass amounted to 93 g ADW (212 g DW) on an area basis, of which the epiphytes contributed 15.5% (28.5% DW), the macroalgal component 12% (32.5% DW) and the seagrass leaves 72.5% (39.5% DW). Aspects of the epiphytic component (e.g., floristic composition, abundance, biomass and production) in monospecific and mixed seagrass communities are discussed.     

Effects of increased temperature and nutrient enrichment on the stoichiometry of primary producers and consumers in temperate shallow lakes

1. We studied the effects of increased water temperatures (0–4.5 °C) and nutrient enrichment on the stoichiometric composition of different primary producers (macrophytes, epiphytes, seston and sediment biofilm) and invertebrate consumers in 24 mesocosm ecosystems created to mimic shallow pond environments. The nutrient ratios of primary producers were used as indicative of relative nitrogen (N) or phosphorus (P) limitation. We further used carbon stable isotopic composition (δ¹³C) of the different primary producers to elucidate differences in the degree of CO₂ limitation. 2. Epiphytes were the only primary producer with significantly higher δ¹³C in the enriched mesocosms. No temperature effects were observed in δ¹³C composition of any primary producer. Independently of the treatment effects, the four primary producers had different δ¹³C signatures indicative of differences in CO₂ limitation. Seston had signatures indicating negligible or low CO₂ limitation, followed by epiphytes and sediment biofilm, with moderate CO₂ limitation, while macrophytes showed the strongest CO₂ limitation. CO₂ together with biomass of epiphytes were the key variables explaining between 50 and 70% of the variability in δ¹³C of the different primary producers, suggesting that epiphytes play an important role in carbon flow of temperate shallow lakes. 3. The ratio of carbon to chlorophyll a decreased with increasing temperature and enrichment in both epiphytes and seston. The effects of temperature were mainly attributed to changes in algal Chl a content, while the decrease with enrichment was probably a result of a higher proportion of algae in the seston and epiphytes. 4. Macrophytes, epiphytes and seston decreased their C : N with enrichment, probably as an adaptation to the different N availability levels. The C : N of epiphytes and Elodea canadensis decreased with increasing temperature in the control mesocosms. Sediment biofilm was the only primary producer with lower C : P and N : P with enrichment, probably as a result of higher P accumulation in the sediment. 5. Independently of nutrient level and increased temperature effects the four primary producers had significantly different stoichiometric compositions. Macrophytes had higher C : N and C : P and, together with epiphytes, also the highest N : P. Seston had no N or P limitation, while macrophytes and epiphytes may have been P limited in a few mesocosms. Sediment biofilm indicated strong N deficiency. 6. Consumers had strongly homeostatic stoichiometric compositions in comparison to primary producers, with weak or no significant treatment effects in any of the groups (insects, leeches, molluscs and crustaceans). Among consumers, predators had significantly higher N content and lower C : N than grazers.     

Plant-herbivore interactions in seagrass meadows

During the past two decades we have gained much insight into the factors that regulate the productivity of seagrass dominated ecosystems, especially those at low latitudes. Here, we review and reassess the importance of plant-herbivore interactions in seagrass meadows, focusing on recent studies that have examined: 1) grazing on live seagrass leaves; 2) consumption of epiphytic algae growing on seagrass leaves; and 3) consumption of planktonic algae from the waters surrounding seagrass meadows. The major conclusion is that, in contrast to what has been reported in much of the literature on food webs in seagrass meadows, a diverse grazing pathway continues to represent an important conduit for the transfer of energy from the primary producers to higher order consumers. This remains true, although in many areas consumption of seagrasses is reduced in an historical context, owing to the overharvesting of many large species of herbivorous waterfowl, turtles and mammals.We also summarize our view of the important gaps in understanding the broadly defined topic of herbivory in seagrass-dominated ecosystems. We suggest that future studies should focus on: understanding the foraging strategies of seagrass herbivores; quantifying the impact of herbivory on seagrass demography, including effects on sexual reproduction, the fate of flowers, and the production of fruits and seeds; and documenting the commonness of compensatory responses to grazing. In addition, the role of chemical defenses in seagrass species remains inadequately investigated. Studies of the roles of nutritional content (as measured by C/N/P ratios) and chemical defenses are also fertile grounds for future studies of epiphytes and their grazers, as are additional experiments to quantify the relative roles of top-down and bottom-up factors as they determine algal growth and abundance. There is also a need to expand the geographical scope of studies of epiphyte-grazer interactions from cold temperate to sub-tropical and tropical waters. Suspension feeders also need to be studied more broadly, with additional experiments required to quantify their effects on water clarity and their ability to fertilize pore waters, and whether benefits from these activities balances the costs of shading and competition for space that can result from both epifaunal and infaunal suspension feeders.     

Response of seagrass epiphyte loading to field manipulations of fertilization, gastropod grazing and leaf turnover rates

This study evaluates the bottom-up and top-down controls on epiphyte loads under low nutrient additions. Nutrients and gastropod grazers were manipulated in a field experiment conducted within a Thalassia testudinum meadow in Florida Bay, FL, USA. The effect of seagrass leaf turnover rate on epiphyte loading was also evaluated using novel seagrass short-shoot mimics that “grow,” allowing for the manipulation of leaf turnover rates. During the summer growing season and over the course of one seagrass leaf turnover period, low-level water column nutrient additions increased total epiphyte load, epiphyte chlorophyll a, and epiphyte autotrophic index. T. testudinum leaf nutrients (N and P) and leaf productivity also increased. Epiphyte loading and T. testudinum shoot biomass and productivity did not respond to a 60% mean increase in gastropod abundance. Manipulations of seagrass leaf turnover rates at minimum wintertime and maximum summertime rates resulted in a 20% difference in epiphyte loading. Despite elevated grazer abundances and increased leaf turnover rates, epiphyte loads increased with nutrient addition. These results emphasize the sensitivity of T. testudinum and associated epiphytes to low-level nutrient addition in a nutrient-limited environment such as Florida Bay.     

Starch grain morphology of the seagrasses Halodule wrightii, Ruppia maritima, Syringodium filiforme, and Thalassia testudinum

Starch grains are a ubiquitous component of plants that have been used in tandem with phytoliths, pollen, and macrofossils to reconstruct past floral diversity. This tool has yet to be fully explored for aquatic plants, specifically seagrasses, which lack phytoliths and are rarely preserved as macrofossils or pollen. If starch grains in seagrasses are morphologically distinct, this method has the potential to improve seagrass identification in the fossil record in such cases where its starch is preserved (e.g. scratches and occlusal surfaces of tooth enamel from seagrass consumers). The goals of this study were twofold: (1) to determine if starch is present in seagrass material and (2) to assess how starch grain morphology differs between different seagrasses.This study focused on four abundant and ecologically distinct seagrasses from the Caribbean: Halodule wrightii, Ruppia maritima, Syringodium filiforme, and Thalassia testudinum. Starch grains were observed in all species except S. filiforme. Grains from H. wrightii are typically observed in side-on orientation, are sub-round to angular, and are fairly small (3-19 μm, end-on). Grains of R. maritima are small spherical grains (4-8 μm) that have a centric hilum and a straight extinction cross with a median angle between the arms of 90°. Grains from T. testudinum are large (9-31 μm, end-on), conical in side-on and round/sub-round in end-on orientation, have a slightly eccentric hilum with an obvious particle, and prominent lamellae.Visual assessment and comparative statistics demonstrate that the morphology of starch grains from T. testudinum, R. maritima, and H. wrightii are significantly different. With more extensive research, there is potential for the positive identification of starch grains from an unknown seagrass. The ability to identify seagrass from starch grains could facilitate the identification of seagrasses in the fossil record and supply information on seagrass evolution and distribution, climate effects on seagrass distribution, and the diets of seagrass consumers.     

Vulnerability and resilience of seagrasses to hurricane and runoff impacts along Florida’s west coast

Many climate change models predict increasing frequency and severity of tropical cyclones (hurricanes) in the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico. To assess this potential threat to seagrass communities in Florida’s Big Bend region, we performed a habitat change analysis based on aerial seagrass surveys performed prior to, and after, the extremely active Atlantic cyclone seasons of 2004 and 2005. To provide a regional context for changes in the Big Bend region, we also compared impacts there with changes in three other West Florida estuaries. Our analysis showed that storm impacts on seagrasses varied along Florida’s west coast. Physical disturbance caused minor losses in parts of Charlotte Harbor and the Big Bend region. However, heavy rainfall in Florida and Georgia associated with Frances and Jeanne combined with winter rains to cause complete loss of 1,500 ha of seagrasses and thinning of another 1,700 ha in the vicinity of the Suwannee River mouth. In Tampa Bay, Sarasota Bay, and Charlotte Harbor, despite localized losses, total seagrass area actually increased between 2004 and 2006. On the other hand, Tampa Bay, Sarasota Bay, and Charlotte Harbor all showed significant, and more pronounced, declines in seagrass cover as the result of another major rainfall and runoff event: the 1997–1998 El Nino event. Our results indicate that light stress, likely caused by suspended sediments, phytoplankton blooms, and dissolved organic matter, resulted in seagrass losses extending up to 40 km from the mouth of the Suwannee River. We conclude that water quality impacts, especially if they are persistent, can be more damaging than physical impacts of moderate (Category 1–3) tropical cyclones. We also conclude that runoff-related impacts on seagrasses vary depending on the timing, volume, and persistence of storm runoff in relation to normal seasonal runoff patterns and seagrass growth in each estuary.     

The loss of seagrass in Cockburn Sound,Western Australia. II. Possible causes of seagrass decline

This paper examines possible reasons for the extensive loss of seagrass in Cockburn Sound following industrial development. Transplanted seedlings survived poorly in Cockburn Sound compared with an adjoining embayment. Altered temperature, salinity, sedimentation and water movement do not explain the death of seagrass over wide areas, and there is no evidence for a role of pathogens. Oil refinery effluent reduced seagrass growth in aquaria at concentrations similar to those at the point of discharge, but could not account for the widespread deterioration observed in the field. Severe grazing by sea urchins was observed on meadows already under stress and does not appear to be a primary cause of decline; caged, transplanted seedlings also deteriorated.Increased light attenuation by phytoplankton blooms may have affected the ddepth to which seagrasses could survive, but would have had little significant effect in shallow water; marked phytoplankton blooms were recorded only after extensive seagrass decline had taken place. Light reduction by enhanced growth of epiphytes and loose-lying blankets of filamentous algae in nutrient enriched waters is suggested as the most likely cause of decline. Heavy epiphyte fouling was consistently observed on seagrasses in deteriorating meadows, as well as on declining, transplanted seedlings, and is known to significantly impair photosynthesis in other systems. Extensive seagrass decline coincided with the discharge of effluents rich in plant nutrients.     

SURFACE INTERACTIONS OF THE EPIPHYTIC MACROALGA HINCKSIA MITCHELLIAE (PHAEOPHYCEAE) WITH THE SHOALGRASS HALODULE WRIGHTII (CYMODOCEACEAE)1

Meadows of Halodule wrightii (Cymodoceaceae) underwent a decline in a tidal flat located at Paranaguá Bay (Parana, SE Brazil). This decline appeared to be related to an overgrowth of the epiphytic macroalga Hincksia mitchelliae (Harv.) P. C. Silva (Phaeophyceae). In order to characterize the type of epiphytism between the alga and its plant host, we compared two samples from the beginning and end of the algal overgrowth via electron and optical microscopes. The investigation revealed that at both sampling periods, there was an epiphytism of type II, which is due to an infection of epiphytes strongly attached to the surface of the host but not associated to any apparent direct host‐tissue damage. The presence of plasmodesmata between the cells of Hincksia only in the late stage of the host–epiphyte interaction indicated a change in the vegetative organization of Hincksia in relation to its host to improve nutrient absorption and distribution through the epiphyte cells. This is the first report on plasmodesmata in H. mitchelliae. The proposed mechanisms with which the algal epiphytes lead seagrasses to death are shadowing by adhesion on Halodule surface and disruption of its osmoregulatory system. Our findings have implications for the conservation and management strategies of seagrass ecosystems.     

The Seasonal Succession of Epiphytic Communities within an Equisetum fluviatile L. Stand in Lake Pääjärvi,Southern Finland

The seasonal succession of epiphytic communities on Equisetum fluviatile was controlled both directly and indirectly by the macrophyte. Decaying macrophytic material supported rich algal growth and biomass accumulation in spring and early summer. Emergence of the macrophytes severely reduced underwater light availability, and the epiphytic algal biomass declined rapidly as a result of both lower photosynthetic activity of the epiphytic algae, and more intensive grazing by invertebrate herbivores. Epiphytic N: P ratios were lower than those in the water around suggesting that either the water was not the only source of phosphorus for the epiphytic algae or the algae took phosphorus up selectively from the water. Low epiphytic C: N ratios suggested a high potential nutritional value for herbivores.     

An assessment of the annual mass balance of carbon,nitrogen, and phosphorus in Narragansett Bay

Narragansett Bay is a relatively well-mixed, high salinity coastal embayment and estuary complex in southern New England (USA). Much of the shoreline is urban and the watershed is densely developed. We have combined our data on C, N, and P inputs to this system, on C, N, and P accumulation in the sediments, and on denitrification with extensive work by others to develop approximate annual mass balances for these elements. The results show that primary production within the bay is the major source of organic carbon (4 times greater than other sources), that land drainage and upstream sewage and fertilizer are the major sources of N, and that landward flowing bottom water from offshore may be a major source of dissolved inorganic phosphorus. Most of the nutrients entering the bay arrive in dissolved inorganic form, though DON is a significant component of the N carried by the rivers. About 40% of the DIN in the rivers is in the form of ammonia. Sedimentation rates are low in most of Narragansett Bay, and it appears that less than 20% of the total annual input of each of these elements is retained within the system. A very small amount of C, N, and P is removed in fisheries landings, denitrification in the sediments removes perhaps 10–25% of the N input, and most of the carbon fixed in the system is respired within it. Stoichiometric calculations suggest that some 10–20% of the organic matter formed in the bay is exported to offshore and that Narragansett Bay is an autotrophic system. Most of the N and P that enters the bay is, however, exported to offshore waters in dissolved inorganic form. This assessment of the overall biogeochemical behavior of C, N, and P in the bay is consistent with more rigorously constrained mass balances obtained using large living models or mesocosms of the bay at the Marine Ecosystem Research Laboratory (MERL).