Shallow Turbid Waters Research Articles

Century-long records reveal shifting challenges to seagrass recovery.

Global losses over the 20th century placed seagrass ecosystems among the most threatened ecosystems in the world, with eutrophication, and associated deterioration of the submarine light environment identified as the main driver. Growing appreciation of the ecological and societal benefits of healthy seagrass meadows has stimulated efforts to protect and restore them, largely focused on reducing nutrient input to coastal waters. Here we analyze a unique data set spanning 135years on eelgrass (Zostera marina), the dominant seagrass of the northern hemisphere. We show that meadows in the Western Baltic Sea exhibited major declines relative to historic (1890-1910) reference due to the wasting disease in the 1930s followed by eutrophication peaking in the 1980s, but have only shown modest improvement despite major eutrophication mitigation, halving nitrogen input since the 1980s. Across the past century, we identified generally shallower colonization depths of eelgrass for a given submarine light penetration and, hence, increased apparent light requirements. This suggests that eelgrass recovery is limited by additional stressors. Our study indicates that bottom trawling and intense recent warming (0.5°C per decade, 1985-2018), which impact on deeper and shallower meadows, respectively, suppress eelgrass from fully recovering from eutrophication. Warming is most severe in shallow turbid waters, while clear-water areas offer eelgrass refugia from warming in deeper, cooler waters; but trawling can prevent eelgrass from reaching these refugia. Efforts to reduce nutrient input and thereby improve water clarity have been instrumental in avoiding a catastrophic loss of eelgrass ecosystems. However, local-scale future management must, in addition, reduce bottom trawling to facilitate eelgrass reaching deeper, cooler refugia, and increase resilience toward realized and further warming. Warming needs to be limited by meeting global climate change mitigation goals.

Light in the Dark: Retrieving Underwater Irradiance in Shallow Eutrophic Waters From AC-S Measurements

Light is essential for primary production and, therefore, its attenuation controls the vertical distribution of plants and phytoplankton over the water column. The diffuse attenuation of irradiance (K) is mainly governed by the attenuation by the water itself and the concentrations of optically active substances (e.g., phytoplankton, inorganic particles and colored dissolved molecules), which makes it an important parameter for eutrophication monitoring. Over the past century, Denmark has had recurrent eutrophication events, with extreme episodic cases where anoxic conditions were observed. Since the 1980’s, eutrophication in Danish waters has been monitored with regards to the diffuse attenuation coefficient of scalar irradiance (Ko) of photosynthetically active radiation (PAR, 400–700 nm), Ko(PAR). However, radiometric measurements in Denmark are difficult in winter due to low solar zenith angle and only few light hours. On top of that, radiometric measurements in the first meters of the water column are highly affected by light refraction influenced by waves, compromising the monitoring of shallow turbid waters as in Denmark. Therefore, we developed a semi-analytical model based on data to from a spectral AC instrument (AC-S, Sea-Bird Scientific) that can estimate the underwater light field and the diffuse attenuation coefficient of downwelling irradiance, Kd(PAR). We tested two distinct approaches based on equations from the literature for estimation of Kd(PAR). The results show that modelled PAR profiles follow the overall shape of in situ radiometric profiles but with smoother profiles, especially in the surface layer (2–5 m). Along with that, the method provided robust Kd(PAR) estimates, that were strongly correlated to the reference Ko(PAR) values from in situ profiles and with low root mean square error (RMSE). Thus, AC-S data can be used to estimate the underwater light field and Ko(PAR). This will make possible to retrieve Ko(PAR) in the absence of daylight and, therefore, allow for environmental monitoring outside the daylight hours, making environmental monitoring more efficient. In addition, the method provides valuable insights into the factors controlling light attenuation.

Fish-mediated zooplankton community structure in shallow turbid waters: a mesocosm study

Turbidity influences the success of fish feeding, but little is known about the difference in the effects of turbidity as a result of organic and inorganic matter. To assess the feeding behaviour of a native fish, Goodea atripinnis, under turbid conditions (up to 120 NTU) generated by Microcystis aeruginosa or silt, we conducted mesocosm experiments for 25 days using 40-L plastic mesocosms. Each tank received two G. atripinnis individuals (predators) with a mean total length 61 mm and cladoceran zooplankton (prey) (Alona glabra, Chydorus sphaericus, Moina macrocopa, Simocephalus vetulus, Ceriodaphnia dubia, and Daphnia pulex) of 100 ind. L−1. Filtered water from Xochimilco Lake was used to fill the mesocosms. Goodea atripinnis had a selective effect on the zooplankton community in the form of a decrease in the number of pelagic cladocerans. The zooplankton composition, which was least affected by the fish, included small-sized (400–480 µm) benthic cladocerans (e.g. Chydorus sphaericus) and copepods (both calanoids and cyclopoids). Prey consumption by the fish was highest under conditions with inorganic turbidity, while in the fishless treatments, small cladocerans (along with calanoids) had a high density. Organic turbidity offered refuge against fish predation to A. glabra, C. sphaericus, Daphnia pulex, M. alburquerquensis and A. americanus, whereas inorganic turbidity allowed only both test copepods to coexist in the presence of fish. The results suggest that fish predation is a powerful factor regulating zooplankton communities, even in highly turbid waters.

Contrasting responses of the coral Acropora tenuis to moderate and strong light limitation in coastal waters

Coastal water quality and light attenuation can detrimentally affect coral health. This study investigated the effects of light limitation and reduced water quality on the physiological performance of the coral Acropora tenuis. Branches of individual colonies were collected in 2 m water depth at six inshore reefs at increasing distances from major river sources in the Great Barrier Reef, along a strong water quality gradient in the Burdekin and a weak gradient in the Whitsunday region. Rates of net photosynthesis, dark respiration, and light and dark calcification were determined at daily light integrals (DLI) of moderate (13.86–16.38 mol photons m−2 d−1), low (7.92–9.36 mol photons m−2 d−1) and no light (0 mol photons m−2 d−1), in both the dry season (October 2013, June 2014) and the wet season (February 2014). Along the strong but not the weak water quality gradient, rates of net photosynthesis, dark respiration and light calcification increased towards the river mouth both in the dry and the wet seasons. Additionally, a ∼50% light reduction (from moderate to low light), as often found in shallow turbid waters in the Burdekin region, reduced rates of net photosynthesis and light calcification by up to 70% and 50%. The data show the acclimation potential in A. tenuis to river derived nutrients and sediments at moderate DLI (i.e., in very shallow water). However, prolonged and frequent periods of low DLI (i.e., in deeper water, especially after high river sediment discharges) will affect the corals’ energy balance, and may represent a major factor limiting the depth distribution of these corals in turbid coastal reefs.

Estimation of underwater visibility in coastal and inland waters using remote sensing data.

An optical method is developed to estimate water transparency (or underwater visibility) in terms of Secchi depth (Z sd ), which follows the remote sensing and contrast transmittance theory. The major factors governing the variation in Z sd , namely, turbidity and length attenuation coefficient (1/(c+K d ), c=beam attenuation coefficient; K d =diffuse attenuation coefficient at 531nm), are obtained based on band rationing techniques. It was found that the band ratio of remote sensing reflectance (expressed as (R rs (443)+R rs (490))/(R rs (555)+R rs (670)) contains essential information about the water column optical properties and thereby positively correlates to turbidity. The beam attenuation coefficient (c) at 531nm is obtained by a linear relationship with turbidity. To derive the vertical diffuse attenuation coefficient (K d ) at 531nm, K d (490) is estimated as a function of reflectance ratio (R rs (670)/R rs (490)), which provides the bio-optical link between chlorophyll concentration and K d (531). The present algorithm was applied to MODIS-Aqua images, and the results were evaluated by matchup comparisons between the remotely estimated Z sd and in situ Z sd in coastal waters off Point Calimere and its adjoining regions on the southeast coast of India. The results showed the pattern of increasing Z sd from shallow turbid waters to deep clear waters. The statistical evaluation of the results showed that the percent mean relative error between the MODIS-Aqua-derived Z sd and in situ Z sd values was within ±25%. A close agreement achieved in spatial contours of MODIS-Aqua-derived Z sd and in situ Z sd for the month of January 2014 and August 2013 promises the model capability to yield accurate estimates of Z sd in coastal, estuarine, and inland waters. The spatial contours have been included to provide the best data visualization of the measured, modeled (in situ), and satellite-derived Z sd products. The modeled and satellite-derived Z sd values were compared with measurement data which yielded RMSE=0.079, MRE=-0.016, and R 2 =0.95 for the modeled Z sd and RMSE=0.075, MRE=0.020, and R 2 =0.95 for the satellite-derived Z sd products.

Distribution patterns of the demersal fish fauna on the inner continental shelf of Suriname

This study aimed to characterise the spatio-temporal patterns of the demersal fish community in the shallow (<40 m) inner shelf waters of Suriname, based on (bi)monthly data for the period February 2012–April 2013. From 148 demersal trawl samples, distributed over 15 locations, 98 fish species were identified. Stellifer rastrifer, Amphiarius rugispinis and Cynoscion jamaicensis represented 50 % of the catches. Cluster analysis revealed three species assemblages, occurring in a nearshore–offshore depth gradient. A coastal fish assemblage occurred in the shallow turbid waters, characterised by muddy sediments with relatively high organic carbon content. A transition assemblage around 27 m water depth marked the shift towards a very different offshore fish assemblage on the deepest sampling locations (34 m), which were characterised by coarser sediments, with clear overlying waters. The coastal assemblage represented the ‘sciaenid community’ of tropical shelves, dominated by Sciaenidae and Ariidae. The offshore assemblage had a significantly lower fish density and diversity, and contained representatives of fish families typical for deeper tropical shelves, such as Paralichthyidae, Triglidae and Lutjanidae. The shift between the coastal and offshore fish assemblage was the most important feature of the demersal fish community, and coincided with a transition between two principal ecosystems: a coastal, river influenced system fuelled by detritus versus an open shelf system based on primary production. Whereas pelagic fishes are known to gain in importance on the open shelf, demersal fishes thrived in the coastal ecosystem, together with a potentially important epibenthic food source, the Atlantic seabob shrimp Xiphopenaeus kroyeri. In Suriname, large demersal (mainly sciaenid) fishes are the main fisheries resource for the artisanal fleet in the nearshore waters below 20 m depth. Because juveniles of commercially important fishes were abundant in our coastal trawl catches, we suggest the shallow nearshore waters have a main nursery function.

A Simple Atmospheric Correction Algorithm for MODIS in Shallow Turbid Waters: A Case Study in Taihu Lake

A simple atmospheric correction algorithm (SACA) based on a “known” empirical spectral relationship at two bands of MODIS sensor has been developed as a modification to improve the MODIS ocean color products in shallow turbid waters. The analysis results for spectral characteristics suggest that 531, 551, 667, and 678 nm are the optimal bands for constructing the empirical spectral relationship of the SACA algorithm. In order to work efficiently with high adjacency effects and water bottom reflectance in shallow inland lakes, the image statistics method is suggested for the SACA algorithm to determine the aerosol scattering contribution. Using the in situ measurements taken in Taihu Lake on October 27 and 28, 2003, MODIS water-leaving reflectance derived from the atmospheric correction were evaluated using the SWIR and SACA methods, indicating that the SACA algorithm produces a superior performance at visible bands but provides a poor result at NIR bands to the SWIR algorithm. To solve this problem, a combined method is suggested: the SACA algorithm is operated at visible bands, while the SWIR algorithm is executed at NIR bands. Results indicate that the MODIS-derived water-leaving reflectance in shallow turbid water within 20% may be obtained using the combined method. These findings imply that the SACA algorithm is an acceptable atmospheric correction method for deriving the water-leaving reflectance from MODIS data in shallow turbid waters.

Differences in foraging ecology determine variation in visual fields in ibises and spoonbills (Threskiornithidae)

Variations in visual field topography among birds have been interpreted as adaptations to the specific perceptual challenges posed by the species’ foraging ecology. To test this hypothesis we determined visual field topography in four bird species which have different foraging ecologies but are from the same family: Puna Ibis Plegadis ridgwayi (probes for prey in the soft substrates of marsh habitats), Northern Bald Ibis Geronticus eremita (surface pecks for prey in dry terrestrial habitats), African Spoonbill Platalea alba and Eurasian Spoonbill Platalea leucorodia (bill‐sweeps for prey in shallow turbid waters). All four species employ tactile cues provided by bill‐tip organs for prey detection. We predicted that the visual fields of these species would show general features similar to those found in other birds whose foraging is guided by tactile cues from the bill (i.e. bill falling outside the frontal binocular field and comprehensive visual coverage of the celestial hemisphere). However, the visual fields of all four species showed general features characteristic of birds that take food directly in the bill under visual guidance (i.e. a narrow and vertically long binocular field in which the projection of the bill tip is approximately central and with a blind area above and behind the head). Visual fields of the two spoonbills were very similar but differed from those of the ibises, which also differed between themselves. In the spoonbills, there was a blind area below the bill produced by the enlarged spatulate bill tip. We discuss how these differences in visual fields are related to the perceptual challenges of these birds’ different foraging ecologies, including the detection, identification and ingestion of prey. In particular we suggest that all species need to see binocularly around the bill and between the opened mandibles for the identification of caught prey items and its transport to the back of the mouth. Our findings support the hypothesis that sensory challenges associated with differences in foraging ecology, rather than shared ancestry or the control of locomotion, are the main determinants of variation in visual field topography in birds.

Spatial and Temporal Variation of Cestode Infection and its Effects on Two Small Barbs (Barbus humilis and B. tanapelagius) in Lake Tana, Ethiopia

Pseudophyllidean cestodes as Ligula have a complex life cycle with cyclopoid copepods as first intermediate host, zooplanktivorous fish as second, and piscivorous birds as final host. We studied the effects of diet, season and habitat occupation on the prevalence of plerocercoid larvae of the tapeworm Ligula intestinalis in two closely related small barbs and the effects of the parasites on the barbs life histories in Lake Tana (Ethiopia) during 1 year. In all affected barbs L. intestinalis caused retardation in gonad development, maturation at reduced size and lower absolute fecundity. Infection rate, averaged over all habitats was significantly higher in B. tanapelagius (10%) than in B. humilis (6%). Below a threshold of 48 mm the infection rate was zero for both barbs, this coincided with a very low proportion of copepods in their diets, increasing up to 90 and 55%, respectively, for their largest size class (81–90 mm). The relatively high infection rate in B. tanapelagius is explained by its obligatory zooplanktivorous feeding behaviour, ingesting a relatively high proportion of infected cyclopoid copepods. This is in contrast with B. humilis, which is a polyphagous species, feeding both on zooplankton and benthic invertebrates. Significant seasonal effects in infection rates were observed. In both barb species infection rates were lower during the breeding season. Only for B. tanapelagius a significant negative correlation was observed between rain fall and infection rate, probably caused by an increased turbidity that decreases feeding efficiency on zooplankton. Habitat type had also a significant effect on infection rate. Barbus humilis showed a much higher infection rate in shallow clear water (10%) than in shallow turbid water (3%), whereas B. tanapelagius showed much higher infection rates in the shallow sublittoral (13%) than in the deeper pelagic (7%). Most likely, birds predate more efficiently on barbs in shallow clear waters than in shallow turbid and deep waters.

The expansion of the south‐western fisheries in late medieval England

This article argues that the expansion of marine fishing in south‐western England from the late fourteenth century to the early sixteenth was part of the maritime sector's critical, but unappreciated, contribution to the rising prosperity of the region. Revenues from fishing represented a substantial supplement to the income of the fisher‐farmers who dominated the industry; promoted employment in ancillary industries such as fish curing; improved the seasonal distribution of maritime work; and stimulated capital investment in ships, nets, and other equipment because of the share system that characterized the division of profits within fishing enterprises. In offering what was probably the chief source of employment within the maritime sector, fishing also provided the ‘nursery of seamen’ so prized by the Tudor navy, and built the navigational experience that underpinned later voyages of exploration.

Laser-acoustic measurements for remotely determining bathymetry in shallow turbid waters

A laser technique is described that has the potential for being developed into an airborne remote sensing system for bathymetric mapping of shallow-turbid waters. This hybrid system consists of an infrared CO2 laser transmitter and a highly sensitive microphone receiver located in the air. The interaction of the laser beam with water generates acoustic signals in water having pulse widths of 20–30 μs and a frequency spectrum peaking between 15–25 kHz. Measurements have been made from both a floating laboratory and an operating ship. These measurements were made using a 5 to 15-J CO2 laser to generate a sound pressure level (SPL) of 185–195 dB re: 1 μPa in water at a depth of 1 m. Measurements made in quiescent waters recorded water depths of 20±0.1 m, while the maximum water depth obtained aboard ship was 13±1 m. The two major problem areas encountered were environmental noise and the angular dependence of the bottom sediment echoes as they penetrated a ‘‘wavy’’ water surface.