Macroalgal Mats Research Articles

Application of color infrared aerial photography to assess macroalgal distribution in an eutrophic estuary, Upper Newport Bay, California

Newport Bay is a large estuary in southern California that is subject to anthropogenic nutrient loading, eutrophication, and hypoxia. Ground-based methods of assessing algal extent for monitoring and management are limited in that they cannot provide a synoptic view of algal distribution over comparatively large areas. The goal of this study was to explore the application of color infrared aerial photography as an alternative for analyzing the changes in the abundance of exposed macroalgae. Three surveys combining remote sensing (color infrared aerial photography) and ground-based sampling to quantify macroalgal mat coverage were carried out in Upper Newport Bay (UNB) between July and October 2005. Airborne photographs (scale 1:6000) collected during daytime low tides, clear skies, and appropriate sun angle were digitized to 25-cm resolution, orthorectified, georegistered, and combined into three mosaic composite digital images: one for each survey. During each aerial photography survey, macroalgal percent cover was measured on the ground by the pointintercept method in a 6.25-m2 area at ca. 30 locations distributed along the water’s edge throughout the intertidal mudflat area. There were three main types of cover:Ulva spp. (green algae),Ceranrium spp. (red algae), and bare surface (mud and mussel beds). To analyze similarities between spectral signatures in the images and cover types, the pixels corresponding to the ground samples from each survey were grouped into clusters based on similarity of their spectral signatures. To establish relationships between spectral signatures in the images and cover as determined from ground data, pixels in each composite image corresponding to ground samples from the same day that were characterized by > 90% of one cover type were attributed to that cover type. Ground samples comprised of a mixture of cover types were used for accuracy assessment. Before classification, each digital image was transformed by the Minimum Noise Fraction Rotation method to remove noise and enhance contrast between the classes. For classification of each composite image, the Spectral Angle Mapper scheme was used: all pixels in each image were attributed to the identified classes and the areal extent of each class was estimated. According to these assessments, the macroalgal coverage in UNB increased from 37% in July to 57% in September to 80% in October, and during this timeUlva spp. replacedCeramium spp. as the dominant alga. This analysis showed that color infrared aerial photography is an effective tool for assessing estuarine, intertidal macroalgal coverage.

Macroalgal standing crop estimate – fast method using ultralight aircraft

A simple and low-cost method for monitoring macroalgal standing crops in shallow water consists of flying over and photographing the areas in an ultralight aircraft to determine the surface area of macroalgal mats. In the proposed method, photos are analysed by AUTOCAD software and mat density is then estimated in the field. Using mat density and bottom surface covered by mats, a rough estimate of the Standing Crop (SC) can be computed. Total of all estimated SCs in a given period is the Total Macroalgal Standing Crop (TMSC). The method was developed to support macroalgal harvesting operations in Orbetello lagoon. SC and TMSC values were needed to decide places and times of macroalgal harvesting and to estimate the boat work required. Applied to ecological studies, this method called Fast Valuation Method (FVM) and is compared with a more time-consuming Accurate Valuation Method (AVM), which calls for extensive sampling and statistical processing. The t-test did not show significant differences between the results provided by the two methods. The results were more similar in winter, when macroalgal growth is slow and wind concentrates the mats. The results may diverge in summer when floating macroalgae are more evenly distributed, with FVM tending to underestimate TMSC. FVM is in any case of potential interest for monitoring studies.

Influence of macroalgal mats on abundance and distribution of dunlin Calidris alpina in estuaries: a long-term approach

Estuaries used by shorebirds during the non-breeding season face many types of human pressure, including eutrophication and consequent occurrence of macroalgal blooms. The impact of this phenomenon on the abundance of dunlin Calidris alpina in the Mondego estuary, Portugal, was explored using long-term, monthly time series, between 1993 and 2003. The data series included total macroalgal cover, macroinvertebrate biomass and dunlin abundance. We observed an increase in dunlin abundance during this period that matched the decrease in macroalgal cover, especially since the promotion of management actions in 1998. We suggest that this increase was related to the increase of some of its main prey species, the polychaete Hediste diversicolor and the bivalve Scro- bicularia plana. At smaller scales, no effect of macroalgae was observed on the proportion of dunlin that chose to feed in alternative habitats (salinas) during the low tide period. However, there was a small effect of the level of algal cover on the proportion of birds recorded on macroalgae at interme- diate levels of coverage (around 25% of total mudflat area).

Does seasonal proliferation of Enteromorpha sp. affect the annual benthic metabolism of a small macrotidal estuary? (Roscoff Aber Bay, France)

Proliferation of fast growing ephemeral macroalgae may occur in intertidal environments and potentially affect the annual gross (GPP a) and net (NPP a) primary production of the system. Seasonal macroalgal proliferation was studied in situ in the Roscoff Aber bay and its influence on the annual benthic metabolism was investigated during low tide. CO 2 exchanges at the air-sediment interface were regularly measured within a benthic chamber in order to calculate in situ net (NPP), gross (GPP) primary production and benthic community respiration (BCR). These measurements were assessed on bare sediments at 3 representative sampling sites and in an additional sampling site regularly affected by macroalgae deposit. On bare sediments, under optimal irradiance daily GPP and NPP exhibited fortnightly fluctuations. Annual net primary production (NPP a) was net autotrophic at the coarsest sites (up to 16 gC m −2 y −1) and net heterotrophic at the muddiest station (up to −22 gC m −2 y −1). On macroalgal mats GPP and BCR increased respectively from 3 to 10 and 3 to 20 times compared to bare sediments. This indicated that, at an hourly scale, the impact of macroalgae deposit on benthic metabolism was very significant. However, at the annual scale, macroalgae deposit and sediment microalgae respectively accounted for 12% and 88% of the annual net benthic primary production indicating that macroalgae proliferation poorly affected the annual carbon budget.

UPTAKE AND RELEASE OF NITROGEN BY THE MACROALGAE GRACILARIA VERMICULOPHYLLA (RHODOPHYTA)1

Macroalgae, often the dominant primary producers in shallow estuaries, can be important regulators of nitrogen (N) cycling. Like phytoplankton, actively growing macroalgae release N to the water column; yet little is known about the quantity or nature of this release. Using 15N labeling in laboratory and field experiments, we estimated the quantity of N released relative to assimilation and gross uptake by Gracilaria vermiculophylla (Ohmi) Papenfuss (Rhodophyta, Gracilariales), a non‐native macroalgae. Field experiments were carried out in Hog Island Bay, a shallow back‐barrier lagoon on the Virginia coast where G. vermiculophylla makes up 85%–90% of the biomass. There was good agreement between laboratory and field measurements of N uptake and release. Daily N assimilation in field experiments (32.3±7.2 μmol N·g dw−1·d−1) was correlated with seasonal and local N availability. The average rate of N release across all sites and dates (65.8±11.6 μmol N·g dw−1·d−1) was 67% of gross daily uptake, and also varied among sites and seasons (range=33%–99%). Release was highest when growth rates and nutrient availability were low, possibly due to senescence during these periods. During summer biomass peaks, estimated N release from macroalgal mats was as high as 17 mmol N·m−2·d−1. Our results suggest that most estimates of macroalgal N uptake severely underestimate gross N uptake and that N is taken up, transformed, and released to the water column on short time scales (minutes–hours).

Dissolved oxygen and nutrient budgets in a phytotreatment pond colonised by Ulva spp.

Net daily budgets of dissolved oxygen (O2), dissolved inorganic carbon (DIC), dissolved inorganic nitrogen (DIN = NH 4 + +NO 2 − +NO 3 − ) and soluble reactive phosphorus (SRP) were determined in a pond colonised by Ulva spp. This pond received wastewater from a land-based fish farm and was used as a phytotreatment plant. Three consecutive 24-h cycles of measurements were performed with 8–14 samplings per day. Water samples were collected at the inlet and outlet of the pond and budgets were estimated from differences between inlet and outlet loadings. The first cycle was started when Ulva biomass was 8 kg m−2, as wet weight. The second cycle was performed after the harvest of ~20% of the macroalgal biomass and the third after the harvest of another ~20% of the remaining biomass. Ulva removal was very fast (<1 h) and samplings for cycles 2 and 3 were started two hours after harvesting, so that the whole experiment lasted ~80 h. When Ulva biomass was at its maximum, the aquatic system was heterotrophic with an O2 demand of 519 mol d−1 and a net regeneration of DIC (2686 mol d−1), NH 4 + (49 mol d−1) and SRP (2.5 mol d−1). The DIC to O2 ratio was an indicator of persistent anaerobic metabolism. Following the first harvest intervention, this system displayed a prompt response and shifted toward a lower O2 demand (from −519 to −13 mol d−1), with a lesser regeneration degree of NH 4 + (11.4 mol d−1) and DIC (1066 mol d−1). After the second Ulva removal the net budget of SRP became negative (−1.0 mol d−1). By integrating these results over the three days cycle we estimated that in order to operate an efficient nutrient control and maintain macroalgal mats in a healthy status the optimal Ulva biomass should be well below ~4 kg m−2 as wet weight. Above this threshold, self-limitation would render most of the algal mat unable to exploit light and nutrients. An efficient removal of nitrogen and phosphorus could be attained through the management of macroalgal biomass only with an optimisation of recipient surface to nutrient loading ratio.

Model simulations of conditions suitable for the establishment of Enteromorpha sp. (Chlorophyta) macroalgal mats

Floating mats of algae regularly appear in shallow bays in many coastal areas around the world. This paper presents a model that simulates the growth of ephemeral macroalgae in shallow bays, and interprets the growth according to physical and biological prerequisites. Model runs emphasized the role of internal nutrient cycling to provide the enhanced nutrient availability needed to ensure abundant growth of macroalgae. Thus, the presence of macroalgae, as well as long water residence times, influenced the nutrient cycling in the bay to support further growth of macroalgal mats.

Macroinvertebrate response to different species of macroalgal mats and the role of disturbance history

Over the last 20 years, the Mondego estuary, Portugal has experienced excessive growth of macroalgae especially in the inner parts of the system, with several algal species implicated. In this study, we compare the effects of morphologically different species, the red alga Gracilaria verrucosa and the green macroalga Enteromorpha intestinalis on macrobenthic assemblages, by a field experiment whereby the biomass of algae was manipulated and the resultant changes in macrofauna abundance evaluated. The experiments were carried out in different areas (a relatively undisturbed sea grass bed and an upstream eutrophic area) experiencing different degrees of overall enrichment. Measurements of sediment redox potential revealed a rapid anoxia with a significant increase in algal biomass after 4 weeks. The effects of macroalgae were different at the two sites, being more marked in the eutrophic area. In addition, the effects of Gracilaria and Enteromorpha were significantly different, with Enteromorpha having a greater detrimental effect for most of the macrofauna, in particular Cyathura carinata, Scrobicularia plana, Cerastoderma edule and Alkmaria romijni. However, three of the most abundant invertebrates ( Hydrobia ulvae, Hediste diversicolor and Capitella capitata) showed significant increases in abundance in weed affected compared to weed-free plots. Gracilaria had less of an impact on macrobenthic assemblages leading to a more enriched community. Between-site differences in overall impact were related to their previous disturbance history.

Effect of green macroalgal mats on burial depth of soft-shelled clams Mya arenaria

Green macroalgal mats are becoming prevalent in many parts of the world, including on important clam-harvesting beaches in SW New Brunswick, Canada. Such mats (Enteromorpha sp. and Cladophora sp.) may be affecting populations of soft-shelled clams Mya arenaria (L.). We inves- tigated the effect of these mats on burial depth of soft-shelled clams in the field (2 impacted sites with high algal cover and 2 reference sites with no algal mats) and laboratory. At impacted sites, burial depth was significantly shallower for clams under macroalgal mats than for those in areas clear of algae. In the comparison of areas clear of algae at impacted sites to the reference sites, clam burial depth was not significantly different; rather, burial depth varied between sites independent of site type. Field measurements of benthic respiration and total sulfides were significantly higher in areas in which algal mats were present than in areas in which they were absent. In an 8 d laboratory exper- iment, clams (4 per aquarium) were placed in sand (10 cm deep) and covered with 0, 2 or 6 cm of macroalgae. Clam burial depth quickly decreased under algae and remained significantly shallower under 2 and 6 cm of algae than in the control (no algae). Near the end of the experiment, we removed the algae, and burial depth quickly increased. At the end of the experiment, clam body mass and dis- solved oxygen at the sediment surface did not differ significantly between treatments, although both variables showed a decreasing trend with increasing algal mat cover. Dissolved organic carbon in pore water at the end of the experiment was significantly higher in the 6 cm algal treatment than in the control and 2 cm algal aquaria. The presence of algal mats clearly affects burial depth of soft- shelled clams. This in turn may have impacts on predator-clam interactions and on the surrounding environment.

Distribution and dynamics of drifting macroalgal mats in Estonian coastal waters during 1995–2003; pp. 260–268

Distribution and dynamics of drifting macroalgal mats in Estonian coastal waters during 1995–2003; pp. 260–268

Benthic algae control sediment—water column fluxes of organic and inorganic nitrogen compounds in a temperate lagoon

Coastal lagoons are a common land‐margin feature worldwide and function as an important filter for nutrients entering from the watershed. The shallow nature of lagoons leads to dominance by benthic autotrophs, which can regulate benthic—pelagic coupling. Here we demonstrate that both microalgae and macroalgae are important in controlling dissolved inorganic as well as organic nitrogen (DIN and DON) fluxes between the sediments and the water column. Fluxes of nitrogen (NH4+, NO3−, DON, urea, and dissolved free and combined amino acids [DFAA, DCAA]) and O2 were measured from October 1998 through August 1999 in sediment cores collected from Hog Island Bay, Virginia. Cores were collected from four sites representing the range of environmental conditions across this shallow lagoon: muddy, high‐nutrient and sandy, low‐nutrient sites that were both dominated by benthic microalgae, and a mid‐lagoon site with fine sands covered by dense macroalgal mats. Sediment—water column DON fluxes were highly variable and comparable in magnitude to DIN fluxes; fluxes of individual compounds (urea, DFAA, DCAA) often proceeded simultaneously in different directions. Where sediment metabolism was net autotrophic because of microalgal activity, TDN (total dissolved nitrogen) fluxes, mostly comprised of DIN, urea, and DFAA, were directed into the sediments. Heterotrophic sediments, including those underlying macroalgal mats, were a net source of TDN, mostly as DIN. Macroalgae intercepted sediment—water column fluxes of DIN, urea, and DFAA, which accounted for 27–75% of calculated N demand. DON uptake was important in satisfying macroalgal N demand seasonally and where DIN concentrations were low. Up to 22% of total N uptake was released to the water column as DCAA. Overall, macroalgae assimilated, transformed, and rereleased to the water column both organic and inorganic N on short (minutes—hours) and long (months) time scales. Microalgae and macroalgae clearly regulate benthic—pelagic coupling and thereby influence transformations and retention of N moving across the land—sea interface.

Vertical heterogeneity in physiological characteristics of Ulva spp. mats

In eutrophic areas, green macroalgae are frequently and for long periods arranged in mats, resulting in a steep light gradient. This study investigates the effect of this gradient on physiological characteristics [tissue nitrogen content, maximal photosynthetic efficiency (Fv/Fm), glutathione levels and redox ratio, absorbance and absorption spectra] of the green macroalga Ulva spp. Mats were sampled during the build-up (June), stationary (July), and decomposing (September) phases of a macroalgal bloom in the Veerse Meer, a eutrophic brackish (salinity 15–20 psu) lake in the southwest Netherlands. Water samples were taken for nutrient analyses. At all three sampling dates, the mats were composed almost entirely of Ulva spp.; in September the mats were in decay and covered with silt and epiphytes. In June and July, total dissolved inorganic nitrogen concentration (DIN) of the water within the mat was significantly higher than outside the mat. Pronounced vertical differences were found in tissue N, Fv/Fm values, total glutathione levels, glutathione redox ratios, and absorbance. In June and July, tissue N decreased from over 2.2% dry weight (DW; N-sufficient) in the bottom layers to around 1% DW (minimum level for survival) in the top layers. Wide-band absorption increased with depth in the mat and throughout the season, probably due to higher Chl a and b and lutein contents. The shape of the absorption spectrum was similar for all layers. The absorption of the silt/epiphyte film on the top Ulva layer was highest; its absorption spectrum (high absorption in the 500–560 nm range) indicates that the film on the top layers of the macroalgal mats mainly consisted of diatoms. In June, Fv/Fm and the glutathione redox ratio of the algae increased with depth in the layer, while total glutathione decreased. Low Fv/Fm values in the bottom and middle layers in September reflect the bad condition of the algae; the mats were largely decaying. It is concluded that multiple growth-limiting gradients occur in macroalgal mats: upper layers suffer from nitrogen limitation and photoinhibition while bottom layers are light limited. The algae in the mat acclimatize to low light conditions by increasing their absorption through increased pigment contents and by higher photosynthetic efficiency during the build-up and stationary period. This study qualifies the glutathione redox ratio as a promising candidate for stress indicator in macroalgae and provides suggestions for its further development.

Impacts of temperature and nutrients on coastal lagoon plant communities

We investigated the independent and interactive effects of nutrient loading and summer water temperature on phytoplankton, drift macroalgae, and eelgrass (Zostera marina) in a coastal lagoon mesocosm experiment conducted from May through August 1999. Temperature treatments consisted of controls that approximated the 9-yr mean daily temperatures for Ninigret and Point Judith Lagoons in Rhode Island (United States) and treatments approximately 4°C above and 4°C below the controls. Nutrient treatments consisted of the addition of 6 mmol N m−2d−1 and 0.5 mmol P m−2 d−1 to mesocosms 4°C above and 4°C below the 9-yr daily mean. Nutrient enrichment produced marked phytoplankton blooms in both cool and warm treatments during early summer. These were replaced after midsummer by dramatic growths of macroalgal mats ofEnteromorpha flexuosa and, to a lesser degree,Cladophora sericea. No phytoplankton blooms were observed in the cool unenriched treatments, but blooms did develop in the mean temperature and warm mesocosms during the second half of the summer that were similar in intensity, though of shorter duration, than those observed earlier in the enriched systems. Macroalgal blooms did not occur in the unenriched mesocosms. Sustained warm water temperatures markedly decreased eelgrass density and belowground production and increased the time interval between the initiation of new leaves, particuarly when the biomass of macroalgae was high. The negative effect of elevated water temperature on eelgrass was significantly increased under conditions of elevated inorganic nutrient input. By the end of summer, virtually all of the measures of eelgrass health declined in rank order from cool, to mean, to cool enriched, to warm, to warm enriched treatments. It is likely that the marked declines in eelgrass abundance observed during recent decades in the Northeast have resulted from an interaction of increasing nutrient enrichment combined with increasing summer water temperatures.

Inorganic nutrient and oxygen fluxes across the sediment–water interface in the inshore macrophyte areas of a shallow estuary (Lake Illawarra, Australia)

Rates of inorganic nutrient and oxygen fluxes, and gross community primary productivity were investigated using incubated cores in July, August and September 2001, in a seagrass meadow of Lake Illawarra, a barrier estuary in New South Wales, Australia. The results indicated that rates of gross primary productivity were high, varying from C = 0.62 to 1.89 g m−2 d−1; low P/R ratios of 0.28–0.48 define the system as heterotrophic and indicate that more carbon is respired than is produced. In order to determine the effect of macroalgae on O2 and nutrient fluxes, measurements were also conducted on cores from which the macroalgae had been removed. The results showed that the O2 fluxes during light incubations were significantly lower in the cores without macroalgae (P 80%). NO2−+ NO3− and o-P fluxes were always very low during the sampling period. The increasing tendency of net nutrient effluxes, especially NH4+ from July to September, is consistent with the increase of the water temperature and seagrass biomasses. However, in September, significantly lower light, dark and net NH4+ effluxes were found in the cores with macroalgae (SA-sediments) compared with the cores without macroalgae (S-sediments). These results support the hypothesis that actively-growing dense macroalgal mats (i.e., algal blooms in September) may act as a filter reducing the flux of nutrients to the water column.

Responses of benthic invertebrates and their avian predators to the experimental removal of macroalgal mats

A field experiment investigating the effects of algal removal on benthic invertebrates and estuarine birds was carried out during September and October 2001. A randomized block experimental design was employed and three treatments were used: sites with algal cover maintained, when necessary by the addition of algae, sites with algal cover removed and control sites that were not manipulated in any way. Total invertebrate numbers increased during the experiment, but species varied in their response to the different treatments. Although Hydrobia ulvae showed the greatest increase in cleared sites after two months, there was no significant difference between treatments. Corophium volutator showed significant colonization of cleared sites during the experimental period, and numbers of Phyllodocemaculata also increased in the cleared sites. Results showed that while more sedentary benthic infauna are less capable of adjusting to the affects of algal clearance, more mobile epifauna and polychaetes show an ability to disperse to cleared sites in a relatively short period of time. Wading birds, however, did not enter the study site in any abundance during the experiment. Black-headed gulls (Larus ridibundus) were significantly more abundant in cleared sites than in algal cover-maintained or control sites when foraging.

Impact of Enteromorpha intestinalis mats on near-bed currents and sediment dynamics: flume studies

The influence of the macroalgal mats of Enteromorpha intestinalis on near-bed current velocities and sediment dynamics was quantified by placing relatively undisturbed cored sediments in annular flumes. Density-dependent relationships were established for E. intestinalis densities; these ranged from 10 to 60% cover when air-exposed (biomass of 4 to 40 g ash-free dry wt m -2 ) and in com- parison with bare sediment (0% cover). There was a significant increase in friction drag with increas- ing E. intestinalis biomass and percent cover, ranging from a mean 18% reduction in current velocities at 10% cover to 56% reduction at 60% cover. The net result of a reduction in currents (depth-averaged between 1 to 12 cm above the bed) and the physical protection of the bed by Enteromorpha was a marked reduction in sediment erosion of 60% at 10% cover to 90% at 60% cover. The presence of E. intestinalis also significantly enhanced sediment deposition measured as a flux from the water column to the bed. At 60% cover the deposition rate was 48% higher than on bare sediment during the first 30 min of slack water (<0.05 m s -1 ). The results show that E. intestinalis has a marked influence on water flow over the bed and the flux of particulate material across the sediment-water interface.

Importance of benthic nutrient regeneration during initiation of macroalgal blooms in shallow bays

Fast growing ephemeral algae are increasingly observed in shallow coastal waters worldwide. This is generally considered a symptom of coastal eutrophication. It has been suggested that the reoccurrence of macroalgal mats, despite ongoing efforts to decrease nutrient loadings, may be explained by the fact that sheltered bays function as self-regenerating systems through benthic regeneration of nutrients. The importance of benthic nutrient regeneration during the initiation of the growth of filamentous green algal mats was investigated in 2 shallow-water bays on the west coast of Sweden. Nutrient regeneration was assessed in situ and in the laboratory, using sediment-water light/dark incubations and pore water distribution patterns. Benthic efflux of inorganic nutrients could supply up to 55 to 100% of the estimated nitrogen demand and 30 to 70% of the phosphorus requirements for the initial macroalgal growth from May to June. However, the availability of the pore water nutrient pool was influenced by the amount and functional type of infauna, and by com- petition from microphytobenthos. Because of the internal nutrient source of the embayments, it is suggested that there may be a significant time lag between a decreased nutrient supply to coastal waters and improved conditions of shallow water embayments.

Re-colonization of anoxic/sulfidic sediments by marine nematodes after experimental removal of macroalgal cover

In shallow beaches of the southern Baltic Sea drifting macroalgal mats frequently accumulate in the summer months, inducing anoxic and sulfidic conditions with devastating effects on most members of the benthic fauna. Our study describes the meiobenthic nematode fauna under such a natural drifting algal mat and its short-term (30 days) response to the removal of the mat, simulating natural displacement. The sediment under algal cover was characterized by high concentrations of total organic matter (3.9%) and sulfide (10.0 mmol l–1). Sulfide was also detectable in the mat itself (2.2 mmol l–1) and in the overlying water column (0.3 mmol l–1). Thirty days after algal removal total organic matter had dropped to 1.7% and sulfide was present only in deeper sediment horizons. Changes in fauna composition of previously algal-covered sediments were compared with ambient reference stations by means of diversity indices and multidimensional scaling ordination. The nematode assemblage under the mat (prior to removal) was characterized by low diversity (H′=0.42) and dominated by the deposit feeder Sabatieria pulchra, the only nematode present in algal-covered sediments in abundances similar to ambient reference stations. Significant changes in community composition in previously algal-covered sediments could be demonstrated after 30 days by means of multiple dimensional scaling techniques. At this time Daptonema sp. and Chromadorita tenuis dominated the nematode fauna and were present in significantly higher abundances than in the reference samples. While diversity indices showed no significant differences between treatments within 30 days, multidimensional scaling proved that differences between treatments were still significant, indicating that the recovery process had not finished. Reasons for the delayed re-colonization of nematodes are discussed.

Effects of a macroalgal mat ( Ulva lactuca) on estuarine sand flat copepods: an experimental study

The effects of a macroalgal mat ( Ulva lactuca) on intertidal sand flat copepods were studied in Jamaica Bay, NY. In 1999, Ulva was removed from paired quadrats in the algal mat. Density and species richness were estimated from sediment cores over seven biweekly periods and compared with Ulva-removal quadrats and with an unvegetated reference site. In a separate analysis at the same site, species composition and density of all copepods within the algal mat were determined from replicated quadrat samples over seven dates. Emergence traps were deployed in all treatments in 1999 to quantify copepod movements from sediments into the water column during flooding tide. Dissolved oxygen (DO) was continuously monitored over several 96-h periods in and above the algal mat. In 2000, three biomass levels of Ulva (0, 100, 200 g DW per m 2) were added to paired quadrats in the unvegetated zone. Density and species richness were estimated over eight time periods. Ulva removal resulted in significant increases in density and species richness, but levels remained lower than at the unvegetated site. Likewise, addition of Ulva biomass produced significant declines in copepod density. However, the differences between low- and high- Ulva-addition treatments were insignificant. Increases in phytal copepods after Ulva addition did not compensate for losses of epibenthic and infaunal species. The overall effect of the Ulva mat was a net loss of copepod density, including the loss of nearly all infaunal species. Anoxia in sediments and within the algal mat are correlated with declines in infaunal species and with escape by some infaunal species from the sediment into the water column.

Role of microphytobenthos and denitrification for nutrient turnover in embayments with floating macroalgal mats: a spring situation

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 30:91-101 (2002) - doi:10.3354/ame030091 Role of microphytobenthos and denitrification for nutrient turnover in embayments with floating macroalgal mats: a spring situation Kristina Sundbäck*, Alison Miles Department of Marine Ecology, Marine Botany, Göteborg University, PO Box 461, 405 30 Göteborg, Sweden *Email: kristina.sundback@marbot.gu.se ABSTRACT: The importance of the microphytobenthos (MPB) for nutrient turnover was studied in 2 shallow bays on the west coast of Sweden, during the period that floating green macroalgal mats generally become established in the area. Sediment-water nutrient fluxes (N, P and Si) and oxygen fluxes and denitrification were measured in light and dark laboratory incubations prior to and during the early growth of green-algal mats (April to June). The importance of microphytobenthic activity and hence, trophic status of the sediment system, magnitude of sediment fluxes and presence of macroalgal mats were shown to differ between the 2 bays. On a daily basis, the MPB decreased the efflux of inorganic N by 30 to 100%, P by 70 to 100% and Si by 10 to 95%. Thus, the MPB appears to efficiently control the availability of sediment-derived nutrients to ephemeral green algae during the period that is critical for the onset of the macroalgal growth. The MPB may even depress the development of green-algal mats, providing that these depend on the sediment nutrient efflux. Microphytobenthic N assimilation was compared with other N-consuming processes, such as denitrification and green-macroalgal N-assimilation. In autotrophic sediments, the MPB was the dominating N-consuming component (1 to 5.3 mmolN m-2 d-1), denitrification (0.1 to 0.7 mmolN m-2 d-1) being a minor N-sink. In the net heterotrophic sediments, however (with no or little net MPB productivity, higher infaunal biomass and hence larger nutrient effluxes), green macroalgal growth was the main N-incorporating process (mean 2.3 mmolN m-2 d-1). This was followed by denitrification (0.3 to 1.4 mmolN m-2 d-1), which removed roughly 20% of the remineralised N. Thus, denitrification may be a significant N-sink in bays harbouring green-algal mats on the west coast of Sweden. KEY WORDS: Sediment · Microphytobenthos · Nitrogen · Denitrification · Macroalgal mats · Sediment-water fluxes Full text in pdf format PreviousExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 30, No. 1. Online publication date: November 27, 2002 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2002 Inter-Research.