Species composition,abundance and biomass of microphytoplankton in the KwaZulu-Natal Bight on the east coast of South Africa

Nearshore marine environments are influenced by an array of variables that can either be land-derived or of marine origin, and nearshore phytoplankton communities may differ in their taxonomic composition and biomass in response to such variables. The KwaZulu-Natal Bight (hereafter referred to as ‘the bight’) is an oligo-mesotrophic, nearshore oceanic environment, that is influenced by both terrestrial run-off and upwelling. A microphytoplankton survey of the bight conducted over several stations and depths and two seasons was conducted in order to ascertain species composition, abundance and biomass. Microphytoplankton abundance was generally low (a maximum of 180 000 cells l–1 was recorded) but differed considerably between sites and seasons. A total of 99 taxa of mainly Bacillariophyceae and some Dinophyceae, Prymnesiophyceae and Cyanophyceae were identified in the present study. In the central bight, higher abundance and biomass were measured in February (wet season), which may be a possible consequence of terrestrial nutrient inputs. In the northern and southern bight we measured higher abundance and biomass in August (dry season). Upwelling was not detected during the study, but an influence of terrestrial nutrient sources was detected at the coastal stations. Turbid conditions were specific to the site near the Thukela River mouth and possibly influenced abundance, biomass and species composition at this site. Historic data on microphytoplankton composition are scarce, but comparisons with surveys from the 1960s reveal that around 60% of the common diatoms recorded then also occurred in the present study. Small taxa [20–200 µm] dominated the microphytoplankton community. Community composition was fairly uniform throughout the bight in both seasons, dominated in general by Chaetoceros species, and on occasion co-dominated by Thalassionema nitzschioides and Dactyliosolen fragilissimus.     

Station Papa Time Series: Insights into Ecosystem Dynamics

The Northeast Pacific has one of the longest time series of any open ocean station, primarily as a result of the weathership station at Station P from the 1950s to 1981. This review summarizes our understanding of the plankton ecosystem for this station and examines interannual variability for the primary producers. The weathership era characterized a period of high temporal sampling resolution with a limited number of parameters being measured. In contrast, the post-weathership period focussed on seasonal sampling (usually three times per year), but a wider range of parameters were measured and sediment traps were deployed to estimate carbon and opal flux rates. The mixed layer depth is shallow compared to the Atlantic Ocean, ranging from 40 to 120 m in late summer and winter respectively. Nitrate, silicate and phosphate are saturating year round with only a few exceptions in the 1970s. Winter and summer Si:N ratios are the same (1.5:1). Ammonium and urea are 0.5 uM in winter and near detection limits (∼0.1 uM) in late summer. Iron is limiting (∼0.05 nM) in late spring and summer for the growth of large diatoms, but iron is co-limiting with irradiance in winter. Chlorophyll and primary productivity are low and show little seasonal variation (about 2 times). Summer chl is about 20 mg m⁻² while primary productivity ranges from 400–850 mg C m⁻²d⁻¹. The f-ratio of 0.25 does not vary with seasons and indicates that primary productivity is fueled by regenerated nitrogen (e.g. NH₄ and urea). Small cells (<5 um) are normally abundant and they utilize regenerated nitrogen produced by the micrograzers; they do not appear to be Fe-limited, but rather controlled by the micrograzers. Shipboard carboy experiments indicate that large diatoms become dominant when iron is added. Therefore top down control is exerted by the micrograzers on the small cells, while there is bottom up control of the large phytoplankton due to low Fe concentrations. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Effect of Salinity and Temperature Gradients on the Distribution of Littoral Microalgae in Experimental Solar Ponds, Dead Sea Area, Israel

Abstract. Several species of cyanobacteria, diatoms and one euglenid alga populated the littoral zone of hypersaline solar ponds investigated over a period of three years. The composition of the microalgae community changed with salinity and temperature. In the shallow marginal zone the diatoms predominated in winter and spring, at salinities of 30–72 g-1^-1 and at temperatures not exceeding 30o°C, whereas in summer, cyanobacteria were the most abundant. In the deeper zone, at higher salinities and temperatures, cyanobacteria predominated throughout the year. The depth limit of algal growth was 60-80cm, where salinity and temperature exceeded 211 g-1^-1 and 48°C.
The most euryhaline and eurythermal among cyanobacteria were Aphanolhece halophytica F remy and Phormidium hypolimneticum C ampbell , among diatoms Nitzschia sp. aff. N. rostellata H ustedt , Amphora coffeaeformis (A gardh ) K utzing , and Navicula massadaea E hrlich .     

A major change in the phytoplankton of a Swedish sill fjord—A consequence of engineering work?

A major phytoplankton change occurred during the late 1930s and early 1940s in Koljö Fjord, a sill fjord on the Swedish west coast. Dinoflagellate cyst concentrations increased tenfold over a short period of time, from hundreds of cysts per gram sediment to thousands; and the species composition of both dinoflagellate cysts and diatoms changed markedly. These changes took place during a period of extensive engineering work at the entrance to the fjord from the Skagerrak. At this time, the entire passage was straightened, a new channel was built in a previously shallow area, and the old connection was closed. This study investigates whether this engineering work could have sufficiently altered the surface-water circulation to bring about the change in the phytoplankton composition. Several mechanisms are explored by which the construction could have influenced the phytoplankton in the fjord. The primary mechanism is probably increased efficiency of tidal-generated surface-water exchange in the fjord, resulting in a larger transport of surface water from the Skagerrak and consequently a changed surface-water environment. This study highlights how engineering work can have a substantial impact on the local and regional marine environment, a factor that must not be overlooked in environmental planning.     

The late Cenozoic diatom stratigraphy and paleolimnology of Tule Lake,Siskiyou Co. California

Lacustrine diatoms are diverse, well preserved and abundant in cores of lake sediment to 334 m depth near the town of Tulelake, Siskiyou County, northern California. The cores have been dated by radiometric, tephrochronologic and paleomagnetic techniques, which indicate a basal age of about 3 million years (Ma) and a nearly continuous depositional record for the Tule Lake basin for the last 3 million years (My). Fossil diatoms document the late Cenozoic paleolimnologic and paleoclimatic history for the northwestern edge of the Basin and Range Province. During the last 3 My, Tule Lake was typically a relatively deep, extensive lake. The Pliocene is characterized by a diatom flora dominated by Aulacoseira solida suggesting more abundant summer precipitation and warmer winters. Increases in Fragilaria at 2.4 Ma and between 2.0 and 1.7 Ma imply cooler summers that correlate to glacial environments recorded elsewhere in the world. Stephanodiscus niagarae and Fragilaria species dominate the Pleistocene. Benthic diatoms of alkalineenriched, saline waters occur with S. niagarae between 100 and 40 m depth (0.90–0.14 Ma). Tephrochronology indicates slow deposition and possible hiatuses between about 0.6 and 0.2 Ma. Overall, the Pleistocene diatom flora reflects cooler and sometimes drier climates, especially after major glaciations began 0.85 Ma. The chronology of even-numbered oxygen isotope stages approximately matches fluctuations in the abundance in Fragilaria species since 1 Ma, suggesting that glacial periods at Tule Lake were expressed by relatively cool summers with enhanced effective moisture. Interglacial periods are represented by variable mixtures of freshwater planktonic and benthic alkaline diatom assemblages that suggest seasonal environments with winter-spring precipitation and summer moisture deficits.Glacial-interglacial environments since 150 ka were distinct from, and more variable than, those occurring earlier. The last full glacial period was very dry. Aulacoseira ambigua characterizes the late glacial and early Holocene record of Tule Lake. Its distribution indicates that warmer and wetter climates began about 15 ka in this part of the Great Basin.Fluctuations in diatom concentration suggests a 41000-yr. cycle between 3.0 and 2.5 Ma and 100000-yr. cycles after 1.0 Ma. In the late Pliocene and early Pleistocene, Aulacoseira solida percentages wax and wane in an approximately 400000-yr. cycle. The apparent response of Tule Lake diatom communities to orbitally induced insolation cycles underscores the importance of this record for the study of late Cenozoic paleoclimate change.The diatom stratigraphy records the evolution and local extinction of several species that may be biochronologically important. Stephanodiscus niagarae first appeared and became common in the Tule Lake record shortly after 1.8 Ma. Stephanodiscus carconensis disappeared about 1.8 Ma, while Aulacoseira solida is rare in the core after about 1.35 Ma. Cyclotella elgeri, a diatom characteristic of some outcrops referred to the Yonna Formation (Pliocene), is common only near the base of the core at an age of about 3 Ma. Detection of local extinctions is complicated by reworking of distinctive species from Pliocene diatomites surrounding Tule Lake.A new species, Aulacoseira paucistriata, is described from Pliocene lake deposits in the Klamath Basin.     

Mid and late holocene diatoms from ‘Water Supply Lake’, Baffin Island,N.W.T., Canada

Diatom analyses from a 40 cm sediment core from Water Supply Lake, northeastern Baffin Island, reveal 67 taxa of primarily benthic forms, strongly dominated by small Fragilaria spp. No major stratigraphic changes are noted in the diatom floral record, which encompasses the last 7000 years. In the uppermost 2 cm, there are increases in: a) F. virescens var. subsalina, b) the ratio of symmetric: asymmetric valves of F. construens, and c) species richness. These are interpreted as reflecting anthropogenic modifications of the lake (chlorination and deepening) associated with its use as a source of water for the community of Pond Inlet since 1979.     

Scale considerations in using diatoms as indicators of sea‐level change: lessons from Alaska

This paper assesses variations in quantitative reconstructions of late Holocene relative sea‐level (RSL) change arising from using modern diatom datasets from different spatial scales, applied to case studies from Alaska. We investigate the implications of model choice in transfer functions using local‐, sub‐regional‐ and regional‐scale modern training sets, and produce recommendations on the creation and selection of modern datasets for reconstructing RSL change over Holocene timescales in tidal marsh environments comparable with those in Alaska. We show that regional modern training sets perform best in terms of providing fossil samples with good modern analogues, and in producing reconstructions that most closely match observations, where these are available. Local training sets are frequently insufficient to provide fossil samples with good modern analogues and may over‐estimate the precision of RSL reconstructions. This is particularly apparent when reconstructing RSL change for periods beyond the last century. For reconstructing RSL change we recommend using regional modern training sets enhanced by local samples. Copyright © 2012 John Wiley & Sons, Ltd.     

An assessment of the utility of regional diatom‐based tidal‐level transfer functions

In order to understand natural sea‐level variability, and to enhance future predictions, accurate and precise estimates of Holocene tidal levels are required. Although the application of diatom‐based transfer functions can yield such data, these work best when underpinned by local training sets. Urbanized estuaries offer little prospect of obtaining local training sets and, instead, the reliability of regional transfer functions has to be assessed. The performance of a published regional (UK) diatom‐based tidal‐level transfer function applied to fossil assemblages from two contrasting core sites in the Mersey Estuary, UK, is assessed using modern analogue techniques and sediment isotope data. We find that, although estimated tidal levels coincide with changes in organic matter source, the frequent lack of modern analogues mean that palaeotide estimates are without basis. This is likely a consequence of the site‐specific nature of diatom assemblages in higher intertidal and supratidal areas in particular, where local factors are expected to exert a greater control on their ecology. This situation may be partly resolved by constructing and applying much larger regional training sets from multiple higher intertidal and supratidal sites (where intact). Otherwise the application of alternative techniques, such as regional foraminiferal tidal‐level transfer functions, may be more appropriate. Copyright © 2011 John Wiley & Sons, Ltd.     

Sensitivity of tidal marshes as recorders of major megathrust earthquakes: constraints from the 25 December 2016 Mw 7.6 Chiloé earthquake,Chile

We present evidence of land-level change resulting from the 2016 M_w 7.6 Chiloé earthquake from tidal wetlands along the southern coastline of Isla de Chiloé, Chile, to test criteria for the detection of low-level, <0.1 m, coseismic land-level change. In order to record coseismic land-level change in tidal wetland sediments, both the creation and preservation thresholds must be exceeded. High-resolution diatom analyses of sediment blocks at two tidal marshes reveal that the 2016 earthquake exceeded the creation threshold and a statistically significant change in diatom assemblage is recorded. In contrast, the preservation threshold was not exceeded and the record of coseismic land-level motion is not preserved at any location visited. After nine months, interseismic and coseismic changes are statistically indistinguishable. The most sensitive part of the tidal wetland is not consistent between research locations, possibly as a result of changes in sedimentation after the earthquake. We compare records of change from great earthquakes in Alaska with the record from the Chiloé earthquake to explore the detection limit. We propose that coastal palaeoseismological records are highly likely to underestimate the frequency of major (M_w 7–8) earthquakes, with important implications for recurrence intervals and assessment of future seismic hazards.     

Preliminary paleolimnological study of rocha lagoon, SE Uruguay

Physical, chemical and biological data from a sedimentary profile of Rocha lagoon (Uruguay) are presented. Analyses of silica remains showed that opal phytoliths, diatoms, sponge spicules and chrysophyte cysts were the most important contributors to biogenic silica. Organic matter, total nitrogen and total phosphorus were used to infer past trophic development. Physical analysis of sediments allowed us to identify five lithological units related to sea level variation and climatic events.