Biofouling formation in the artificial reef structured space

Abstract

The paper focuses on the possibility of using artificial reef structures (ARS) to control biofouling and focuses on the spatial organization of vegetative canopies formed in the ARS, which is relevant for solving the problems of cultivation of hydrobionts on solid substrates. ARS is a kind of stockade made up of cylindrical vertical structures (d_i = 0.33 cm, l_i = 20 cm) that are placed on a horizontal plate (S_o = a₁ · a = 900 cm²). The stockade covers space (V_g = 0.018 m³) partly occupied by vertical structures (ΣV_i = V_i · n, where n is the total number of axial structures on the module horizontal surface), while the rest of it remains free (V_f = V_g – ∑V_i). Three possible densities were considered for the arrangement of the said vertical structures (n/S_o), and namely: 544, 3956 and 10 678 pieces per m², while the concentration of their surface area within the ARS (C_s = (∑S_i) / V_g ) was set as 0.056, 0.295 and 1.106 per cm. For 43 days (from May 27 to July 9 2014), the ARSs were kept in the offshore strip of the Sevastopol Bay (Black Sea) at the depth of 2 m. The water temperature in the sea ranged from 23.3 to 25.6 °C. Samples were taken every 6–8 days. In the composition of the biofouling, diatom algae were found to be predominating in the mass, while also present were sprouts of multicellular algae and, of animals, hydroids, bryozoans, spirogbis, and larvae of balanuses were also observed. On the 7^th day of exposure, 51 species of diatom algae were found on the surface of the ARS with a loose (544 pieces per cm²) and dense (10 678 pieces per cm²) structures arrangement, with only 15 species making a significant (> 5 %) contribution to the total number (n_i, %) and biomass (W_i, %) of the community in certain sections of vertical structures (Achnanthes longipes, Amphora hyalina, Berkeleya rutilans, Cylindrotheca closterium, Entomoneis paludosa, Haslea ostrearia, Licmophora abbreviata, L. hastata, Neosynedra provincialis, Nitzschia sigma, N. tenuirostris, Parlibellus delognei, Pleurosigma elongatum, Proboscidea insecta, Trachyneis aspera). The values of the Sorensen – Chekanovskii (K_sc = 0.7) and Stugren – Radulescu (P_sr = -0.077) coefficients indicate a very close similarity between the systematic composition of the communities being compared. As the fouling density value changes in vertical structures (W/S_i) during the period under consideration, four characteristic stages can be distinguished. The first, by convention, is completed on the 7^th day of observation, and an approximately equal fouling density (0.51–0.91 mg (dry weight) per cm²) is found to have been created by this time on the tops of the structures, regardless of the density of their structures, and it differs significantly (0.03–0.57 mg (dry weight) per cm²) in the middle part of the ARS. The second stage (7^t–21^st days) is characterized by low rates of biomass increase per unit of the surface colonized (0.003–0.08, 0.25, -0.17 mg (dry weight) · day^-1 · cm^-2) and by relatively low values W/S_i (0.36–2.23 mg (dry weight) per cm²). The third stage is characterized by a rapid increase in the fouling density (0.30–0.75 mg (dry weight) · day^-1 · cm^-2). The maximum W/S_i (3.09–9.07 mg (dry weight) per cm²) is reached on the 29^th and 36^th days of exposure. The fourth, final stage is characterized by a decrease in W/S_i, this being the period of partial “disintegration” of the previously formed fouling community. The paper analyzes in detail the vertical distribution of the dry biofouling biomass (W/Si) along the axial structures with different density of their arrangement on the 7^th, 14^th, 21^st, 29^th, 36^th and 43^rd days of the experiment. The general picture of the vertical distribution of W/S_i on the 29^th and 43^rd days was found to be similar. With the increase in the density of vertical structures arrangement, the maximum fouling biomass shifts towards the upper boundary of the ARS. In loosely arranged structures, the maximum biomass is located in the middle part of the canopy, while in not so densely arranged structures (3956 pieces per cm²), the bulk of the biomass (83.5–73.8 %) is concentrated in the upper half of the canopy, while in densely arranged structures, in the upper 2-cm layer (63.9–79.3 %). The paper also considers the relationship between the biofouling dry mass concentration throughout the inhabited space (C_w = (∑W_i) / V_f ) and the concentration of the ARS physical surface with respect to the upper ARS (1) layer and the entire volume of the reef structure (2) in 20-cm high structures on the 29^th day of the experiment ((1): C_w = -0.232 + 7.136C_s, R² = 0.99; (2): C_w = 0.084 + 2.652C_s, R² = 0.93). It shows that in 20-cm vertical structures an increase in the value of C_s is accompanied by the increased screening effect produced by both structure elements and biofouling – a process which leads to the growth of biofouling agents being partially checked by the insufficient inflow of light and biogenic elements and, accordingly, to a “shortage” of biomass in the given volume of the structure space.

Keywords: biofouling, phytofouling, colonization, phytocenosis, diatoms, artificial reef, vegetative canopy, inhabited space, communities inhabiting hard surfaces